If this happens, search your entire computer for files with the name of emWave.emdb. The chances are that the original installation might not have been done to the default location, or was done under a different user. If so, searching for the emWave.emdb will find the original file.
If you find that there is more than one .emdb database file, you can access it directly from the emWave program. The file may be very small, possibly because it only has one User in it or a limited amount of Session data.
You may want to consider deleting the small database file, while keeping the larger one.
Log in to emWave, enter your registration number again and that should be the last time you'll have to do that.