Unless you had backups of your library, I'm not sure if at this point you'll be able to recover the previous play counts.
What I'd suggest going forward is that you install the portable edition of MB, if that's not what you're currently using. (and then update to the latest version
This installation type allows you to have all your MB files under one parent folder. So backing up MB will be really easy (in preparation for unforeseen events like this)
As far as my experience goes, you could have lost your play counts as a result of two things:
1. Maybe the songs were moved/renamed without using MB's built-in File Organizer (that results in MB not knowing where the files are. So data like play count is lost along the way)
2. Maybe the library you were using before also got moved/renamed/deleted...it's the only thing to me that explains why you got that "choose music library" dialogue box on startup
As for the empty "date added" fields, I honestly have no idea what may have led to that. Maybe someone else here does and will comment on it.
I haven't been here long enough to know if this is something that gets reported frequently. But if this was a technical issue with MB, it would have been picked on long back.
I don't recommend using the current library for future use...since it's missing things like dates added.
Consider my suggestion at the start of this message of starting again on a clean slate with a portable version that gives you 'more control' over your MB files.
Besides the two scenarios mentioned above, this is not something you can expect to happen again.