I used my Dell M65 to zero out a separate hard drive that I had, so changed the BIOS to boot from the CD-ROM and used a disk wiping utility from there. Everything went fine, but when I was done and put my normal hard drive back in, the computer would turn on but was unresponive. I can't get BIOS to come up, and it will no longer boot from the utility cd, or even from a windows installation disk. The power light comes on for the laptop, but the screen never flickers and I can't elicit a response of any kind no matter what keys I press.
First (not to state the obvious), make sure your HDD is installed properly and that it plugged into position correctly. Second, if that still doesn't do the trick, remove the HDD and see if it will at least let you into the BIOS. We can attempt to rule out whether the HDD is the problem.
Another thing to consider is the system's built-in diagnostics processes (I have an M65). The system "may" have detected a change in state and is running diagnostics when you first power on. If you let the system sit for awhile after powering up (give it 30 minutes), does it do anything (other than sit there)?
It is possible that you shorted the SATA connector when swapping HDDs, but isn't too likely. Don't give up hope yet.