Agreed on making sure you have the right revision ROMs. The A3000 was a weird one. Especially with the V1.4's and ROM tower. I forget the details. But there are a lot of them. I'm sure the information is still out there, if you look enough.
As for it not powering back up when set back to the old ROMs... Agreed, check the seating on any processor card, if you have one. Also, check the seating on the power cable. And make sure you didn't accidentally change any other settings, too.
Nothing you've done should cause any damage, provided you weren't extremely unlucky. (ESD, etc...) But these old machines get really finicky. Takes a ton of patience and re-seating things once they've been disturbed.
