Try removing the hard drive and then re-seating it on the connector.
If that does no good, Google the computer make/model you have and find out how (which key) to enter BIOS.
Look around in the BIOS settings and find the option to reset the BIOS settings to default and/or alternatively set the internal hard drive containing the operating system as the first boot device (again through the BIOS settings).
Another reason you can get the error message you've posted is when the hard drive has failed - - but carry out the suggestions from Cloudy1 first.
I have to say that in most cases where friends of mine have had that message on the screen (which means your PC is trying to perform a network boot 'cos it can't find any other boot device) I've had to replace the hard drive for them, only rarely has it been something else.