Not receiving Messages on macOS I'm not receiving new messages in the Messages app on macOS (High Sierra). I do get notifications of new messages there (delayed many minutes from when I receive them on iOS). And I can send messages from macOS (which appear immediately on my iOS Messages and at the recipient). But no new Messages of any kind appear in Messages on macOS.

Is there a way to totally and completely reset Messages on macOS so that it works again?

(Note that despite failing to be of any use, Messages on macOS renders the machine unusable when running by maxing out CPU usage. So perhaps a clean resent of Messages in necessary to fix that as well?)
I’m not sure what steps you have taken to resolve the issue so I’ll go least invasive to most invasive.

-Restart your computer
-sign out of messages and sign back in
-if the issue persists set up a test user account and attempt to replicate the issue. Below is a link on how to do so.

https://support.apple.com/kb/PH25796?locale=en_US

-if the issue replicates in a test user than you have a system wide issue involving the messages app.Next steps would be to make sure your data is backed up and perform an in place install using the recovery partition (hold down command and r on start up and choose install high sierra).
Being that your CPU usage is high for this app, install may be the best step.