(1) see if the disk drive is found. Go into BIOS. Look at installed devices. You should see a hard drive. If not the hard drive has failed, a cable has fallen off, etc. Reply if the hard drive is not visible in the BIOS, or if you need help finding the hard drive in teh BIOS. Unfortunately this is the most likely cause of the message.
(2) If the hard drive is found then make sure the boot order includes the hard drive. This is a menu in the BIOS. Some BIOS also allow you to choose which device to boot, sometimes this is invoked by hitting F12 during power up. Check your manual, try to force the PC to boot from the disk if the disk is found.
(3) There is a record called the MASTER BOOT RECORD that is sometimes corrupted by viruses or other problems. It can be repaired easily. If the disk drive is found, but you cannot boot off it (you get the message above) then google 'repair MBR WIN7'
(4) Your Dell probably has a bootable recovery partition on the hard disk. Your manual will tell you how to boot off the recovery partition. If the disk is there, and is working then you have the option of booting off the recovery partition and doing some recovery work. Read the manual. Some of the recovery operations jsut restore system software. Others wipe all the data from the PC and make it like factory new.