Bummer..Well first off never unplug/plug in stuff with power on. Probably know that now though. Its really unsafe to be rooting around in the game with the power on.

Checking fuses would not hurt but unlikely they are bad.
First thing i'd try is re-seating the ribbon cables a few times to make sure they are making a good connection. And make sure all other connections are present and attached, with the power off of course.
Like the DMD cpu board which is critical it has power. Should not have damaged anything if you got lucky but next time shut er down to remain safe and mitigate any damage
Cheers and be careful my friend

OK. Thats the main power in for the cpu so it sounds like now with the additional info in your last post that a fuse may indeed be bad. Check the power supply and the like. 12v and 5 volt and ground are all on that connector

Also make sure its plugged in right. The index pin may be missing and the connector is off by 1 pin now. Also to check the fuses for sure. remove them and use a DMM set to continuity or beep test. They can look good and be bad.

After you know the power is good once the fuses are checked and voltages are checked at test points we can see if the cpu boots. Something may have been damaged on the MPU board and its preventing game boot.

his game still displays garbage on the display so I think the HV is fine and suspect the game is no longer booting after the main power connector to the cpu was removed and plugged back in with the power on.

checking the fuses and testing the voltages will tell us the tale. Hoping for a bad fuse for him. Sigh, tough lesson when all you want to do is play pinball.
Hang in there you'll get er

Thats true , also when too high or off voltages etc....BUT his main board is not booting as well. No sound or anything. Also he did not remove the HV pulg with the game on only logic power plug cn17.
Only low voltages present there

No sweat you will get it. As it all started with the cn17 off on cycle with the power on, the trouble more than likely lies with the supply voltages
Easy fix.