Blinking cursor on boot

Sometimes a computer will fail to start up and you just end up with a blinking cursor on a grey screen.  It looks as if something drastic has gone wrong - but it may not be.  One cause seen recently is that there is a USB memory stick or USB disk drive plugged in and the computer has tried to boot from it. 

Unfortunately, and for reasons that are not clear, you don't get a sensible message when this happens as you might for say, a floppy disk - "Operating System not found - non-system disk or disk error, press any key to continue". So, don't panic, just remove any USB devices and try the start-up again.  To prevent this happening in future you need to go into the computer BIOS and reset the boot sequence - refer to your computer's instructions to do this.