|This happens to every one once. You probably protected the hole drive including the Windows Directory. You can not do this because you block the SYSTEM user (which is what windows boots up with until you login) from accessing the Windows directory which contains all drivers and DLLs that are need to boot Windows.|
Now I hope you did a backup because the only way to get your computer back is to reformat the drive and reinstall Windows. You could try putting the drive into another computer and see if you can access the files. It will ask you for your user name and password. put in your old computers name with a \ and your login name (i.e. MyComp\JohnDoe) and put in your password and see if it will decrypt the files for you.
Good luck and remember next time only encrypt your My Documents not the hole C: drive.