Probably you have to check all connections .. I have build dozens of computers and it has happened to me many times that ;
floppy / harddisc cable was not properly connected
graphic/sound card was not firmly in its slot
memory modules / cpu
Did you forget that your graphic card needs to be connected with a power cable? I think my brother also didnt get past the post screen because of that.
Another very nasty problem that happened to me once is that the mainboard was in contact with the case. I had forgotten to use plastic washers which produced some sort of short circuit. The computer wouldn't POST but it all went away once I had installed the mboard a second time with the washers.
Keep checking and you will finally get it running .. in most cases the hardware doesnt take damage, fortunately, but care is necessary.
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