Can you get to the boot options? This is usually accessed by tapping on the F8 key during startup click here (Dell's may have a differnt key to press to bring these up - your manual may have this info).
If yes, choose the 'Last Know Good Configuration' option to begin with.
If no joy with that, try the 'Repair Your Computer' option.
Details for all the above in the link provided.
If you need to take it further with a recovery/reinstall, then click here. Don't attempt these if the hard disk has important data, emails, etc as they will need to be backed-up first. G