I opened Mail app, but nothing comes — no list of mailboxes, no messages, nothing.

Did you add any email accounts to Mail?

Add an email account to Mail on your Mac - Apple Support

What happens if you open the Message Viewer from the Window menu (cmd-0)?

If that opens the message viewer, then don't close the message viewer when you want to quit the app.

XYZ543_a wrote:

How do I gain access to my email?

To add to that same vein—Option Command N opens a New Viewer Window

if that is the only issue.