Hi julissafitmom,
Thank you for using the Apple Support Communities. Messages is a great app, and we'll be happy to help out if you're unable to open it. It sounds like you've done some troubleshooting already.
When you say you reset your Mac, what process are you referring to? Do you mean you restarted it, or did you completely reinstall macOS? Knowing this would help work out some good next steps.
Also, do you get an error when you open Messages? If so, what does it say?
As a starting point, we suggest booting your Mac into safe mode to see if the issue occurs there. That will help to rule out a few possible software causes. This link shows How to use safe mode on your Mac. Once your Mac reaches the desktop in safe mode, see if Messages opens, and then click Apple menu () > Restart to get back to normal and test again.
Feel free to reply back here with the results. Be sure to include the answers to the questions above, along with your macOS version from Apple menu () > About This Mac.
Take care.