Can I assume you also tried reset, to no avail?
Do you have the capability and equipment to test internal connections?
The first thing to check is power. If you're really careful, you can check the voltages from the expansion port. The bottom leftmost pins are -5V, +5V, and +12V. The wide pin on the top left is the ground pin. Just don't short a power pin to the copper springy guides or to another pin. Stick a post-it note or something on the guide to insulate it.
If the power is good, the problem is likely to be in the oscillator circuit. Possible failure points include the crystal itself (sometimes they can quit oscillating until spiked again, say, by turning the system off and back on), U22 (two logic gates that the oscillator uses), U8 (the clock divider) and Q2 (apparently a clock amplifier).
Edited by ChildOfCv, Mon Apr 29, 2019 11:52 AM.