I have mmssms.db but the telephony.db is lost, can I get back my SMS messages in any way? For reasons unknown, my SMS messages disappeared today. I have a rooted SGS2. I've discovered that SMS messages are stored in mmssms.db and I was lucky enough to find a backup file. However, no such luck with telephony.db, and now when I replace the empty mmssms.db with my old one, the messaging app removes it and creates an empty one.

Is there a way to plug my old mmssms.db in there? Or maybe reconstruct the old telephony.db if that could help in some way?
SMS messages in smartphones have generally been stored in SQLite3 databases for a long time now. If the database is not damaged in any way, you should be able to view the contents and maybe export them to some CSV or HTML format with some type of SQLite database browser software.

Here's one you can give a try: http://sqlitestudio.pl/

Otherwise, I know that a data recovery company that specializes in cellphone data recovery would probably be able to recover the messages for you. Some have means of carving out text messages even if the database is damaged.

I don't know that there is a good way to "plug it back in" to your phone though. It's interesting it sees it and then creates an empty database. There may be some identifying marker that it sees doesn't match, perhaps with something related to the last OS update.