I thought I had lost a file somehow until I looked in the directory where I had it using Windows Explorer and noticed that all the constituent CAESAR files were still "open", even though I had closed (or thought I had) the file. So I tried opening the _a file because it was displayed when I selected pre 5.0 version option in the drop down box, but not surprisingly got an error message that says the file was created under version 0.0. Can I repair/retrieve/fix this somehow?

That is hard to say. "Version 0.0" indicates a corrupt input file. "Maybe" we can recover this. Send the _A and _P files to techsupport@coade.com. Make sure you indicate what version of the software you're using.

Did you have a power failure, or something strange that would have caused CAESAR II to just quit (abort) on you?

Ok thanks - we have had issues earlier today with some analysts not being able to launch CAESAR but nothing that I know of occured recently.

I've experienced this when my windows explorer had the directory the CII file is in opened. Somehow the combination view directory content and CII saving files doesn't seem to work. This gave the same result as described above.

You can also try to recover the last revision of the input. In Windows Explorer type "%temp%" into the address bar. From this location in the (Explorer) tree, go up a few directories to "Application Data". Navigate down into "Application Data" to "CAESAR II v.5.10". Here you'll find a subdirectory for each job you have worked on. Go into the directory for the job in question, and grab the latest _A file from there.