I'm having the same issue and I have a critical update to PHP pages that needs to get uploaded in the next day or two. I have v. 2017.5 Release 9878 Build. My Webassist extensions are gone, nothing shows up in Databases, I can't recreate recordsets or server behaviors. I'm completely lost and panicked! Do I need to redownload the extensions?
UPDATE: I just tried to reinstall the MySQLi extension and I got the message that it was already installed (but it's not there in the top menu - why?) So I tried to delete and reinstall, and got the error message: "Cannot find the data for the database; therefore extension will not be installed". Now what?