DELL desktop computer suddenly not booting.

I went to start my DELL desktop computer today, but I could not get past the initial DELL logo on the screen. I could also see the boot options with the F# keys, but I could not use any because the computer was not responding to the keyboard input. I turned everything down and unplugged my external hard drives. Next the computer turned on, but I got a “missing NTDLR” message. What should I do next? What was the problem in the first place?
