Last night I ShutDown normally

This morning, during the Boot Process, after the POST I am at an A prompt confonted with:

"The following file is missing or corrupt: Command.Com

Type the name of the Command Interpretrer eg C:\Windows\Command.com "

I have no idea what is required and I cannot get into Safe Mode because the Startup Menu only offers the choice 'Normal' and I arrive back at the above

Does anyone know what I am expected to type in?

I have a feeling that this is something to do with the mbr and that there is a way to fix that but I don't know how

Thanks