Try this first:
1. Unplug the power cord from the pc
2. Press the power button to drain any leftover charge from the system
3. Remove the CMOS battery and press the power button again
4. Replace the CMOS battery, plug the power cord back in, and try starting the pc again.
If it works, then your system probably overheated while you were playing fallout. Check your temps, especially on the cpu, make sure nothing's overheating.