Thanks guys for your help.

Danny I followed your instructions and all is now okay.

Michael, this is what I was doing when the problem occurred.

I drew two concentric circles(was going to create a tube), selected both then tried to extrude them. Nothing happened and I tried again. Same thing.
I think that's when I noticed the top and left windows were blank.

As you suggested I tried to repeat these actions but everything worked as it should.

Will let you know if it happens again.

Thanks again.

Cheers
Mal