I can't login to my xubuntu. After I enter my login details at the prompt it just shows the prompt again!
If your password is definitely correct, it may be a corrupt user profile preventing you from logging in. Try booting in to recovery mode, then select the root shell option. Once you're at the root console, create a new user by running the commands below:
Once you've created your new user, reboot the machine and try to login. If this works then you can either use this new account and copy any files you need from your old home folder. Or you could try backing up your old user's files, removing the old user (and its home folder), then recreating the user.
Are you sure you're typing your password correctly? (Are you sure you're typing your username correctly?) Spelling and capitalization count.
Make sure CapsLock is off.
If all else fails, reboot your system into "recovery" mode, and issue the command