Messages

Start with this...

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes or more.

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

Are you running any VPN, Anti-Virus, or Cleaning apps?

In Messages>Preferences>iMessage>Settings, does it show who you can connect as, Enabled & signed in?

If so try signing out then back in.

Did that work?

So it's solved now???

Ah, great to hear, take care my friend. :)