I have a 720ef s/s built under oven. How do i set the clock

You obviously have a power loss somewhere. The oven will not work unless a time is set on the clock. I would suggest you have problem with the feed to the cooker which an electician should be able to check it out. If anything else is wrong on the cooker, it shouldn't reset the clock.

The double oven will only work if a time is set on the clock. Any time will do, you just have to set a time. The grill/top oven is independent of the clock. If the clock is faulty espare can sorce them for you.

Both the top and bottom oven will only work if a time (any time) has been set on the clock. Setting the time operates a contact to switch the power through. The only exception is the grill. This will work even if the time has not been set. So if the oven controls/lights and heating work, to me it means that the clock/timer is half working and the only thing that isn't is the display.
I haven't had the clock/timer to bits but a fellow blogger has said that in some timers there is an electrolytic capacitor that controls the display. When this fails as they are renowned for doing, the dispaly mal functions. If there is one then Maplin have some - VHR41W/43W and DT64U depending on value etc. Let us know how you get on.
If you need a new clock/timer they are around £80 from espares.

Sorry no. you have a defective control panel

I don't know if this will help. For the 720ef , this is what you need to do.
There are five buttons on the clock panel and from left to right they are:- Minute Minder, Cook time, Ready Time, Plus and Minus.
TO SET THE TIME OF DAY
Press the Cook Time button and the Ready Time button simultaneously. Use the Plus and Minus buttons to set the time.(24hr clock).
TO SELECT AN ALARM TONE.
Press the Minus button to listen to the first tone. Release it and press it again to listen to the second tone etc (3 tones)
Releasing the button after the tone has sounded will automatically set that tone.
SETTING THE MINUTE MINDER.
Press the Minuter Minder button and either the Plus or Minus buttons simultaneously to set the time.
You can cancel the timer at any time by setting it back to 00.00 by using the Minus button.
After setting, the display will revert to the time of day, but by pressing the Minute Minder button again, the display will show the remaining time.
If you want all the information, then it is best if you supply a vaild email address and this will give you access to my BT Digital Vault, where you can have a complete copy of the handbook that I have scanned in.

Try powering everything off at the main switch, waiting a few mins, switching back on and set the clock. Make sure the oven clock is in manual mode and switch the oven on and see what happens.Let mw know.

When you say stopped, is it just the oven fan and the cooling fan is still running? Is the oven control knob still lit and is the main oven LED on?
Is the time set on the clock and is the clock set to manual?

When you turn the oven on, does the background cooling fan come on at all?
Also, is the time set on the clock, and everything else set to manual running.
(With some manufacturers the oven will not work if the power has been removed for any reason. Until the time is set on the clock, power is witheld from the oven).

does anyone know were i can get a replacement top oven inner glass for my stoves heritage 720EF ?

To set the time of day. Press the cook time button and the ready time button simultaneously (2nd and 3rd from left). Use the plus and minus buttons to set the required time.