Try logging in as a normal user (you did create a normal user account when you installed linux, didn't you?) and open a command line. Type:
your root password
mv /HOME /root
That is only if you had changed /root to /HOME in the first place.
Then you should be able to log in.
You shouldn't really change the names of any system folders or anything connected with root. Always log in as a normal user, and use the commands 'su' or 'sudo' to perform necessary system tasks. This will save you A LOT OF problems.
edit: you can also use an installation disk (go into rescue mode) or a live cd to change the name back to /root