when your doombuilder 2 crashes (probably because you dared to ctrl-z and roll the dice), stop everything you are doing and follow these simple steps:

step 1) take a deep breath, calm down a sec. don't worry, you didn't lose your map. and even though you obviously weren't saving (/enough), don't take this out on yourself. we can fix this together

step 2) navigate to your AppData folder, it should be in your user directory, but it'll be a hidden folder. You can enable showing hidden folders in your folder options (hit alt -> tools -> folder options -> view -> Show hidden files, folders and drives)

step 3) now in the appdata folder, go to Local -> Temp.

step 4) inside that folder is a buncha junk. there will, however, be one or more folders inside with 8 character names (handy example http://i.imgur.com/T6A5Nrw.png those 3 in the middle)

step 5) go inside them (if there's more than one, check the date created/modified of the files inside, and find the most recent)

step 6) there'll be a bunch of files with the extension .snapshot, these don't interest us, we want the one file with the extension .temp

step 7) copy this file somewhere safe, and rename it to *.wad

step 8) open it up in doombuilder and voila

NOTES: it'll change the map name from MAP01 or whatever to TEMPMAP. you'll need to change this back before you can test.

I think that these are created every time you test, but I might be wrong. I test pretty often, even just to check out architecture, so mine is always not far from when it crashed. at most 10 minutes work. however, I dunno how up to date it is if you don't test often. maybe it has nothing to do with it? who knows.

EDIT: scifista confirms that you don't need to test. thanks :) this is good news
oh and step 9) thank me later