Dear rusty,
I think this is bad luck and you can't do anything inside of Pegasus Mail. I just tried deleting a folder (of course a test folder...); the folder seems to be deleted on operation system level. I recommend the following to you (but without any guarantee....):
0/ Try to avoid loading new files or doing changes to your entire file system.
1/ Make a - or better two - complete Backup(s) of your Pegasus Mail system AND your Mailbox.
2/ Use a tool to unerase the file on the operation system level. (Pegasus Mail folders consist of two files: .pmi and .pmm Unix mailboxes consist of a .mbx and a .pmg-file)
3/ As soon as you have unerased the file start Pegasus Mail. Please do not download or send mails in this session.
4/ Create a new folder in Pegasus (store no mail in it).
5/ Terminate Pegasus Mail
6/ Check (e.g. with motepad) in your mailbox directory the file HIERARCH.PM, look for the filename of the new folder; it's the last string in the first quoted field and starts with UNX or FOL (e.g. UNX056E1 or FOL06B44).
7/ Write down the file name of this new folder
8/ Delete the files of the new folder
9/ Rename the files of the old folder to the name of the new folder.
10/ Restart Pegasus Mail.
If this "restore" doesn't work please restore the backup you did in step 1.
For the future: Consider a regular and robust backup of your mails.
Yours,
Markus