iMessage Plugin Gone from Messages
Oddly enough this usually has a simple fix to it. Yes you may have to reinstall MacOS but that can be done over top of what you already have with no danger of losing anything. However another approach might be in order.

Download the latest combo update from Apple here. Run it, and reboot when done. it is likely to fix the issue. 

If not then boot into recovery mode and reinstall your OS. You will then likely have to re-apply any subsequent updates again to the base system that was just reinstalled.