Hello irose_57,
If you're unable to load email in the Mail app the first thing to check is if you're using any sort of antivirus software or VPN. If you are, try disabling it then test the Mail app to see if messages still won't load.
Testing your computer in safe mode is also a good way to determine if any third party software may be causing the issue. You can find the steps to start in safe mode here: How to use safe mode on your Mac
Once it's started in safe mode just test out the Mail app and see if the messages are able to load or not when the computer is started in this mode.
If messages still won't load, you can try removing and re-adding the email account in the Mail app.
Remove email accounts in Mail on Mac
Add email accounts in Mail on Mac
If things still aren't working properly, we'd recommend contacting Apple for assistance: Get Support
Cheers!