Not receiving Messages on macOS
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.