Gaborone
You have very likely blown "U20" on the MPU. (this is a SUPER common outcome when working on a machine with the power on).
If U20 is soldered to the board don't attempt to replace it yourself unless you have EXCELLENT PCB soldering experience or you will cause more problems than you fix. Send the board out for professional repair. Fit an IC socket to the U20 position when repairing as this will help make future replacement easy.
Please take Lloyds advice and study PinWiki - it will help you fix 90% of problems in your machine.
Reno, NV
