Computer will not boot

Recently my company had a client that could not get his pc to boot up. Since I was doing remote support I was not physically in front of the pc in question. I had him try to get into safe mode however each key combination that we tried to get into the boot menu or the bios setup were unsuccessful, alls that the computer would do was to freeze on the Dell screen. Since we could not get into the boot menu or bios I thought that it might be a motherboard failure. I was also not able to remote into the pc from the server and could not ping it. The only option that I could come up with was to have him ship the computer to our office so that we could look at it and perhaps remove the hard drive to recover the data. It was only after setting up a new pc for him that I learned that he had an external hard drive attached to his pc, once he unplugged that the pc was able to boot, apparently it was trying to boot to the external drive which of course did not contain any OS