Everything was working fine, but all of a sudden, I can't open any of the tutorial files. When I click on them I get a message that some application is not properly installed and there is no default program associated with this file type. Heeeeelp
If they were opening file and now they don't it's most probably a file association issue. Windows keeps a list of file extensions and the program to use to automatically open such files, when the user double-clicks on it. Some of these file associations are set by programs, during installation, but ultimately, the user controls and can change those (the process may be a bit different depending on which version of Windows you are using - search Windows help on your system for File Associations, for specific details). Sometimes, these file associations get corrupted and all the user has to do is reset the program that will be used to automatically open a certain file type. Many times, such corruption may occur when the user installs an updated version of a program that was associated with a certain file type.
AFAIK, the tutorials are simply pdf files that are opened by Adobe Acrobat Reader (which comes pre-installed on most systems, nowadays). Did you by any chance install an updated version of ACReader? If so, that's where I would start looking. This may very well be the culprit. Of course, if you want to totally rule out/in a problem with the TL program, just use your Windows explorer to locate a pdf file on your system and then double-click on it. Does the file open? If it does, then it's probably something in the way TL tries to open the files. If it doesn't, thens it almost certainly a file association issue. Search windows help and find the information on how to reset them.