First, look at the last line for the specific error message (right above the OK button). If that error is listed on the list of error messages page, then follow the instructions for that specific error.
If not, note that Frets on Fire v1.2.512 was released with some bugs in it, some of which also result in this error message.
Note: Don't forget that when uninstalling FretsOnFire the configuration file will usually remain on your computer. That's why prior to installing another version you should delete that configuration file named fretsonfire.ini