Thanks for your help on this. When I turned on the computer today and ran Cubase the sound was coming out as normal. (I have installed the ASIO4ALL driver).
However after playing an audio file from a web page via a browser Cubase then stopped making any sound. When I opened the Windows 7 volume mixer Cubase had disappeared from the list of programmes running. I exited Cubase then restarted it and it then reappeared in the volume mixer and sound started coming out. Is it the case that Cubase loses its connection to the sound output if another programme takes this over and Cubase then can’t reestablish a connection?
I assume this is what was causing my problem a couple of days ago.