Its because you are booting in graphical mode...which starts the X-Server. So when you switch to console, you don't shut it down...you just switch to another terminal that's all.
Hence, when you type 'startx' you're trying to start X-Server again...and it can't. Just use your ctl-alt and either f5,f6 or even f7 (try em til you find it) and you'll go back to it.
There's a way I think to have multiple X-windows running, but I don't know how you'd do that (try google) ...I also don't know WHY you'd do that really...