If a Linux system goes into a command line shell then it usually indicates failure to load some device, usually your graphics card.
Depending on your flaver of Linux you can try typing either:
X
startX
That will try and start the Linux GUI (Grapahical User Interface) or Linux style Window interface where you can use point and click to navigate your way about.
When you go into a shell it is normally because your run level is set to single user mode or the Windows equivalent of Safe Mode and it means you do not have network support amongst other things. Unless you are an expert you probably won't know how to fix whatever is causing the problem. The important thing is to try and get an error message to find out what went wrong and you can try typing this at a commond prompt:
dmsgMore info about this here:
http://en.wikipedia.org/wiki/DmesgTo exit from a shell, type:
exit
To shutdown your system properly from a command line, type:
shutdown