Without the information requested in my previous response, I can only give you a general answer however, I hope this helps.
These issues usually occur due to power surge/power cut.
To solve the issue, please follow these instructions:
You will have to leave the appliance completely unplugged or turned off at the mains for around 1 hour.
Then plug it back and reset the clock, it should work.
However, if the issue appears again then I am afraid, the clock control is faulty and will need replacement.
Alternatively, you may call Rangemaster at 01926 457628 to book repair.
If there is anything else I can help you with, please let me know.