Make sure that you check for simple answers first. If your BIOS was set up or changed to boot from floppy or CDROM first, then check that their isn't a floppy or CDROM left in your drives. If there is, the PC may be trying to load the boot loader (NTLDR) from a floppy/CDROM where one doesn't exist.
The next think I would check would be to go into the BIOS and make sure that my hard drive wasn't turned off by accident. If the hard drive isn't showing up in the BIOS, then you might have a hardware issue. Open the case and uplug the data and power cables on both ends and plug the back in firmly and try again (basically the first item on the Dell trouble shooting list