It sounds like all that has happened is that your reply window has been shrunk.

Next time that window appears, instead of clicking maximise, try and hover your mouse over the edge of the window and drag it bigger. If I'm right, when it gets big enough you'll recognise that it is actually your reply window.

Somehow Pegasus has remembered a tiny setting for that window. After you resize it, Pegasus should remember the new setting.

I have been using Pegasus for years and have never encountered this problem before. When I click to reply to a message, whether in response to a message I have already opened or directly from a list of messages, instead of opening the reply message window, a small task bar is produced on the bottom left hand side of the screen. This only has the options of minimise, maximise and close. (If I click on the minimise icon in the bottom left hand task bar, it toggles to restore up but then if I click on restore up, it toggles back to minimise.)  The only way I can reply to a message is by clicking maximise on this small tool bar, replying to the message on the full screen and sending it. After sending it, the new mail list returns but this is also maximised but it can then be restored down using the top right hand corner icon. My partner also uses Pegasus on the same computer and this isn't happening to her.