A couple things come to mind.
* For the A2000, try opening the case and pulling out any cards besides your 'SCSI card+hard drive'. Maybe one of the other cards is stopping the boot process. You can't hurt anything buy pulling out cards and doing a power up, just don't plug/unplug cards while the PC is powered on.
* Look for a label on the SCSI card, does it say anything like Commodore A2091, GVP HC, Trumpcard, ICD? This will help further down the line.
* When you say the A2000 turns on then off immediately, do you mean you see a Workbench screen or flashes of color? Screen colors are a built-in POST(PowerOnSelfTest) if you list the last color you see perhaps we can narrow down the boot problem.
* Depending on when you need your data you can pull the 'SCSI card+hard drive' and take it to the a Commodore club meeting or VCFED show. Someone there should be able to plug it in and tell you if it is good.
* If you're headed to Las Vegas anytime in the next 6 months, bring it along and I can test it for you.