This morning I noticed that the Pegasus window was gone. I restarted the program, and rebooted the computer but nothing changed.

What happens is this: When I double-click on the Pegasus shortcut, the start-up window appears, but when it closes, the main window is not visible. Clicking on the icon at the bottom of the screen does not make it visible. However, the program must be working somewhere in the background, because i can hear "new mail messages" when I send test mail. Also, I can right-click on the icon in the notification area and get a "new message window"; however, I cannot do anything with it, except closing it (no clicks accepted; no keyboard events processed).

I guess something happened in the OS (Windows 10) that got Pegasus to store a faulty window state, but I do not where or what. Anyone have a clue?

I still think I am using some kind of hybrid of Pegasus; I installed 4.80 some months ago, but immediately had a nasty crash pasting something when composing a message. I then downgraded back to 4.73, but still had some problems with pasting in new messages, so I assume a part of 4.80 did survive the downgrade. For my problem in January, I assumed this information was not relevant (a correct assumption, it turned out), but this time I am not so sure.

It sounds like the window position got corrupted resulting in it being positioned off screen. Close all programs except Pegasus Mail. Now right-click on the windows toolbar and select "Cascade windows" or "Show windows side by side". This should bring it back on screen. You will then have to resize and reposition the Pegasus Mail main window to your liking.

The use of multiple monitors is a common cause of this when Pegasus Mail is closed while on the second monitor. It's window position is then off screen when only a single monitor is used. Also, an abend during shutdown of Pegasus Mail can cause a window position issue.

This is unlikely. Most problems are associated with configuration files. These are stored in the mailbox directory which version changes do not touch.

Thanks for the help; I have now Pegasus working again! However, the items you suggested did not work, but "Show desktop", followed by "Show open windows" did the trick — after that, an empty window header bar appeared in the top left corner of the screen, and that could be manipulated to show the main window. After closing down three copies of Pegasus that used up 100% of the CPU I could again start Pegasus normally, and now everything seems in order again.