Why were you trying to boot from CDs?  After you did, did you change the boot sequence back around in system setup if you're trying to boot from the HD (especially if you don't have a bootable cd in the cd-rom drive)?

Go into you System Setup when you reboot & make sure you have your boot devices listed as floppy first, Hard drive second.

reason I say this is because I got this problem once before when I was trying to boot from a CD, but couldn't, so when I went back into the BIOS to switch things back around, I ended up with the order being: 1) floppy 2) CD rom 3) HD.  This resulted in the problem you described, and making the HD the second boot device fixed it since the PC could boot from the HD after checking the floppy for boot files.

Look at the LEDs on the back of the tower to see if they give you a clue.  All 4 should be green if the PC's in a healthy state.  You can look up the diagnostic LED codes at Dell's site if the lights are not all green, and this will help you troubleshoot the problem further.

Anyway, I'm wondering if your boot sequence in system setup is wrong.  If the boot sequence is correct, then it's possible that the bios can't find bootable files on your HD, meaning that your HD may need to be reformatted, or that it's hosed.

I'd check all connections in your PC, though, before coming to that conclusion.  Particularly, check all cables going to the HD to make sure they are secure.