I did the Lion upgrade last night. Now I can't open Viewer Window in mail, nor can I see incoming mail. I can't open other windows either, aside from Activity (that is constantly trying to connect) and Connection Status, to know that my primary POP email account is broken. Preferences will not open. The Quit command will not work.
I tried the dock icon replacement, the Systems Preferences open/close, rebooting, shutting down and waiting for things to cool off, etc.
I'm on a 5-year-old 20" iMac 2.16 GHz Intel Core 2 Duo, and just found out that this model suffers from video chip overheating. Would video and Mail behavior problems be related?