Check to see that your custom database file is still there and not missing or corrupt.
Use notepad.exe to view, not excel. Excel can resave the file in an incompatible format.
Usually if the file is missing, creating a new custom frequency set will just start a new file. So sounds like the file may be corrupted.
It should be a standard text file, one record per line, ending with a blank line.
When you save a frequency set, it writes it to the file I mentioned above. If that file is corrupt or missing, it will not have anywhere to save the information too.
For more details, please check: