If the columns aren't in the list shown in preferences -> add your own columns then the columns are gone from the database.
If you are very lucky the data might still be in the backup OPFs that calibre creates for each book. Go to the folder for a book that you know should have data in one of the columns and look at the file "metadata.opf" in a text editor. Search for the lookup key for the column (as in #mumble). If that key is there, and if the data is somewhere around the key, then you might be in luck.
To get the data back (as much of it that still exists), you must destroy your library and recreate it. This is not for the faint of heart. Your mileage will vary. It might not get much data back. You may waste hours for little, or even negative, benefit.
The steps:
1) MAKE A BACKUP of the entire library. Almost certainly something will go wrong at least once in this process, and you really really want to be able to put everything back.
2) After making the backup (and preferably testing it) and with calibre not running, in the original library remove the files metadata.db and metadata_db_prefs_backup.json.
3) Open a command window, go to the folder that used to contain the metadata.db file, and run the command
Code:
calibredb --library-path=. --really-do-it
calibre will chug along for some time, rebuilding the database from the information in the metadata.opf backup files.
Please note that the process can lose data. If some metadata.opf files are missing or corrupt then those books will not be added. Information in the DB not directly related to books, such as user categories and saved searches, will be lost. Other bad things can happen. If the result isn't satisfactory, restore your backup.
Recovering a database this way was intended for use as a last resort. You should decide whether this is one of those cases.