The hard drive has failed.  If the system is under warranty, contact Dell with the  error and validation codes and they'll arrange a replacement drive.

If the system is out of warranty, you'll need a new hard drive of the same type and physical size you now have, plus your recovery media (which you should have prepared when you first received the system).  If you need further assistance choosing a replacement drive,  we'll need to know the exact model number of the system you have (i.e., Inspiron N5110, 5521, etc.) -- not just Inspiron 15.