This usually means that your Boot file is corrupted or the HDD has failed and needs to be replaced.
Remove any USB Mass Storage devices from the laptop (Flash Drives, HDD, Camera etc). Restart computer and press F12
and do the Dell Diagnostic
Also, On another computer You can download the ISO image for Seatools
in my signature, and burn the image to CD using IMGBurn
also in my signature. Boot off of the newly created CD and run the Short and Long tests on the HDD. If either of these tests fail, the HDD needs to be replaced.