unity & toolbar at top disappears

Sometimes the unity bar on the left hand side of screen and also the bar at the top of the screen ,with date,file edit ,user account,system settings etc ,does not appear after I log in to Ubuntu with my password,so I cannot access anything,I have to put in the live cd for Ubuntu 12.04(ISO disc) go to try out Ubuntu on the disc ,take out disc (restart & hope unity returns with tool bar again at the top of screen) does any body know why is this and is there some kind of a fix for this say by trying to open a terminal somehow ,its a bit of a nuisance when it does this,any help appreciated to get me over this glitch,Thanks in advance J.Peters.