Batteries good, but can't enter numbers. - Texas Instruments TI-84 Plus Calculator

If you cannot enter numbers that means memory is full or command line is full. You must make room for new commands and new data.
I suggest to
Reset the memory or clear the data, equations, graphs etc. that may still be in calculator memory.

TO RESET THE RAM (any programs you typed in) will be deleted

1. Press [2nd] [ ] to open Memory management tool
2. Press 7 number key to select [7:Reset]
3. You will be asked to select [1:All RAM] or [2:Defaults]
4. Select [1:All RAM]
5. Press [2] to Reset.
6. After memory is reset calculator displays Done.

1. Press [2nd] [ ] to open Memory management tool
2. Select [2: Mem Mgmt/Del..]
3. If you know what kind of objects you want to erase then select any other appropriate item but [1:All]
4. If you do not know what is taking up space select [1:All]
5. A lising of all objects in memory is displayed
6. When thery are empty, the Lists L1 to L6 are 12 bytes long. Letters are 18 bytes long.
7. To delete an iten in the displayed screen, highlight it (by moving cursor to its level) and pressing the [DEL] button.