sounds like you just swapped the Hdd's over and they don't like the new mobo.
Does a boot floppy work or boot CD?
what OS?
are you changing parts back and forth and it previously worked doing this?
i frequently exchange parts from 1 computer to anotrher, but recently i encountered a problem. when the computer starts up it goes through the standard memory test, ands then lists the primary master/slave and seconday master/slave, but after that it freezes. i have an amd k7 1.2 ghz with 256mb of ddr ram. any ideas?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic