DIAGNOSIS This is typically a control panel lockout, not a hardware failure. The Safety Lock (often called Child Lock) feature is almost certainly enabled, disabling all buttons except Stop/Clear and Start/+30 Sec.
FIX To disable the Safety Lock, press and hold the Stop/Clear button for about 3 seconds. You should hear a confirmation beep and see any display indicators for the lock turn off. The number pad should now work normally.
IF THAT DIDN'T WORK If holding Stop/Clear does nothing, try unplugging the microwave from the wall for a full 2 minutes. This performs a hard reset on the microcontroller. Plug it back in and test the buttons before setting the clock.
HEADS UP The Safety Lock can get engaged accidentally by leaning on the panel. It's a good feature, but it causes a lot of panic when you just want to heat up some soup.