'Mail' does not retrieve messages . . .

Suddenly my 'Mail' program does not 'Get Mail' when I clik the button or choose the command in the menu. Nothing happens, no sign of downloading anything.

But the red dot on the Dock icon shows I have 8000+ unread messages.

I haven't added any new software or upgraded the OS for a number of days. Worked OK this morning.

I've tried restarting both the program and the computer.

Any ideas?

Thanks.