Error beeps? If so count them and post back in here the number and grouping of the beeps. No beeps? No video? No post messages? We're gonna need a little more to go on here. It could be anything from a loose cable on the HDD or power switch to a mobo failure or PSU that's smoked.

Do a "Basic Components Test" well thats what i call it probably has "real"name. Take out EVERYTHING apart from the CPU(you can leave in LED signal leads because these can be a pain to get back in if there not labeled), just connect the Power to the motherboard, and video card (if the video is not controlled by the motherboard) and just 1 stick of ram if you have 2. turn on. enter Bios (usually DEL key). if you get video everything "basic-wise" is fine, also check that your monitor isn't faulty if you get into bios load default settings then turn off things like "quick boot" and "quiet boot" and similar stuff. next connect just the HDD, go into bios again, look around for HDD config (or similar it's different on all Motherboards) there should auto-detect or similar hit that, exit and save. restart, should find your HDD if not may have to just keep restarting 2-3 times. if it boots, then shut down and plug in all your other stuffs, CD drive, any PCI cards etc. then should be fine. then you can turn on the stuff your turned off in bios, if it fails the "Basic Components Test either your Video card, motherboard, CPU, or ram is faulty. however if your ram is faulty your machine should give a series of beeps. hope that helps :)

I had a similar problem a few months ago and I had to replace my motherboard, so if following the above advice doesn't work I would suggest that you may have the same problem I had.

Go into setup(cmos) and see if your new hdd is recognized, if not then replace your motherboard if you are sure everything you did replacing the drive was correct.

It could be your motherboard. Well i'm no computer expert but i guess it's the motherboard. Better yet take your PC to an expert so he can fix the problem for you.