Re: Cannot login
Don't know about the shutdown problem;possibly you have to add a kernel parameter to the grub boot line, but somebody else might be able to advice better there.
For the login, have you tried <ctrl><alt><Fn> (n being a number between 1 and 6)? You should end at a console where you're prompted for your username and password. Enter your username and password (the latter will not give any indication of what you type). To shutdown from there issue sudo shutdown -h now; replace -h by -r to restart.
Another thing to check
I'm not sure about the GUI login screen of lubuntu. are you prompted for a username or do you select a user from a list with the option to use 'other' (which will allow you to type a username). In the username field, type your password; this will show the password and that way you can verify that it is typed correctly. Why I suggest this is that the keyboard layout might be wrong (e.g. UK instead of US).
If you don't make backups of your important data, your data is obviously not important to you.