The cursor blinks as a checkered black square and keys don't work

The manual can be downloaded from here: http://college.cengage.com/mathematics/latorre/calculus_concepts/4e/assets/students/gcp/latorre_4e_grcalcguide.pdf

First instance of discussion is on page 1.

Is the cursor checkered? If so, there is no more space on the entry line. Use the DEL key to shorten it or clear it. Clear the RAM also.

It seems to me that the cursor (regardless of its shape) is always blinking. What is significant is the shape it has.
Full rectangle: Overwrite character at the right.
Underline : Insert mode, inserts next key press to the left of its location.
Checkered rectangle: No entry is possible because command or memory is full.
A in reverse video, in ALPHA mode.

If the cursor is not allowing you to type anything that means either the calculator available memory is full or that you have exceeded the number of characters in a single command.
Clear calculator memory or just clear all entries (history) or split a complicated command into multiple shorter commands.

According to the manual, this indicates that either the maximum number of characters have been entered at a prompt or memory is full.

You posted under Leviton 6124 programmable timer.
If this is not your timer, add a comment
Leviton 6124 manual:
http://waterheatertimer.org/images/6124-manual.pdf

If you have different timer, look at link below to help identify timer.
Leviton Timers:
http://waterheatertimer.org/Leviton-timers.html

Leviton 6124 timer instructions:
Timer comes with plastic protector over display >> remove protector to see display.
Large button is manual override.
After timer is wired, push override to make sure Load turns ON and OFF.

Timer has 3 small buttons located below manual override used for programming
Display is divided into 24-hours with am times on left and pm on right.
24 Hour day is divided into 1/2 Hour segments
Each 1/2 hour must be set for ON or OFF

Start by pushing left-side event-cursor-advance-button and it scrolls down each 1/2 Hour.
Once you arrive at 1/2 hour > make decision if you want timer ON or OFF for that half hour.

Press event-set-reset-button in center to set ON-OFF for that 1/2 hour.

Press center event-set-reset-button and solid black rectangle fills in for that half hour.
Solid black rectangle means timer ON for that 1/2 Hour.

Once solid square is selected, press event-cursor-advance-button and cursor advances again and starts filling in black rectangles for each 1/2 Hour period.
Keep filling in black rectangles for all times you want timer ON.

When you reach 1/2 Hour period you want timer OFF, then press center event-set-reset-button so that no black rectangle is visible for that 1/2 hour.
No black rectangle means timer is OFF for that half hour.

To set current time, use right-side time-advance-button.
Press and hold time-advance and time cursor advances down through the am and then pm hours.
Stop cursor at current time, and timer is ready.

Leviton 6124 timer instructions:
Timer comes with plastic protector over display >> remove protector to see display.
Large button is manual override.
After timer is wired, push override to make sure Load turns ON and OFF.

Timer has 3 small buttons located below manual override used for programming
Display is divided into 24-hours with am times on left and pm on right.
24 Hour day is divided into 1/2 Hour segments
Each 1/2 hour must be set for ON or OFF

Start by pushing left-side event-cursor-advance-button and it scrolls down each 1/2 Hour.
Once you arrive at 1/2 hour > make decision if you want timer ON or OFF for that half hour.

Press event-set-reset-button in center to set ON-OFF for that 1/2 hour.

Press center event-set-reset-button and solid black rectangle fills in for that half hour.
Solid black rectangle means timer ON for that 1/2 Hour.

Once solid square is selected, press event-cursor-advance-button and cursor advances again and starts filling in black rectangles for each 1/2 Hour period.
Keep filling in black rectangles for all times you want timer ON.

When you reach 1/2 Hour period you want timer OFF, then press center event-set-reset-button so that no black rectangle is visible for that 1/2 hour.
No black rectangle means timer is OFF for that half hour.

To set current time, use right-side time-advance-button.
Press and hold time-advance and time cursor advances down through the am and then pm hours.
Stop cursor at current time, and timer is ready.

Hello,
Between the x`squared key and the log key there is a key with an x with a raised small figure of a solid square. That is the key you use to enter arbitrary exponents.

1. If calculator is in MathIO ( a Math indicator appears on the upper band of the screen), enter the base of the power (here 2). A number 2 with a raised empty square appears.The cursor will blink in the raised square. Enter there the exponent (here 12) and press [=]. Result 4096 appears at the bottom of the screen.
2. If in LineIO (no Math indicator appears on the narrow band at the top of the screen), press 2 and then the key I talked about above. On the screen a number 2 appears with a hat and a left parenthesis. Type in the exponent (here 12) , the right parenthesis and press [=]. The calculator should display 4096.000...

Hope it helps

Hello,
You should specify your calculator and avoid posting the same question multiple times.)
Here is the solution for FX-300ES

Between the x`squared key and the log key there is a key with an x with a raised small figure of a solid square. That is the key you use to enter arbitrary exponents.

1. If calculator is in MathIO ( a Math indicator appears on the upper band of the screen), enter the base of the power (here 2). A number 2 with a raised empty square appears.The cursor will blink in the raised square. Enter there the exponent (here 12) and press [=]. Result 4096 appears at the bottom of the screen.
2. If in LineIO (no Math indicator appears on the narrow band at the top of the screen), press 2 and then the key I talked about above. On the screen a number 2 appears with a hat and a left parenthesis. Type in the exponent (here 12) , the right parenthesis and press [=]. The calculator should display 4096.000...

Hope it helps

I fixed mine by using tiny pieces of tinfoil. Just take the back plate off (held by 2 screws under the battery cover). Then remove the circuit board (one screw on top). You'll find that the Square Button is in deed different, using 4 rubber contacts that apparently wear out. So, just cut some tiny pieces of tinfoli to cover each of the 4 black contacts. Glue them in place with a dab of silicone... Viola!!!