Boot up with a boot disk, then type at the "boot:" prompt:
or whatever your root partition is actually on. This will get you to the command line--a start. From here you can change your runlevel to whatever you want in your /etc/inittab file. Runlevel 3 will get you to text login, and runlevel 5 will get you to GUI login. There's lots of posts here at LQ regarding the runlevels. Do a search and you'll find lots of posts on exactly how to switch your runlevels around. Hope this helps.