Which control are you on?
On newer P100 and 200 controls, you could use a windows utility to undelete the files.
Most of the older controls you will need to get out your pen and paper and start writing it down. If power goes off before this occurs, you are out of luck.
I have been able to somewhat recover lost files by dumping memory sectors to the RS232 port and capturing the sectors onto a PC, but that removed all formatting, so it was rather ugly, but did save some typing and potential numerical errors. The data is still there, but the file has been marked for reusable space, so as soon as you install something else, the memory sectors can be erased/overwritten.
Trying to recall the procedure...finding the sector will take some poking around as well.
Went something like this:
PATCH MD1: (defaults to screen, use ",TT: " to send out the 232 port or maybe "CN0:" will work also)
>S 1 (sector 1 will display to screen)
Q (quits displaying sector 1 and allows new sector input)
Q (second time will quit Patch mode)
You will get something that looks like the attached picture. Like I said, not pretty, but better than nothing.