Recently I have been using the Creation Kit to create things. I save regular, I was in the middle of building a house when the kit crashed. As I relaunched the kit, I selected Skyrim and the mod name as the data files. Whenever I try to save, I get the following message "File 'Bradleys Mod.esp' is a master file or is in use".

How am I suppose to edit my mod and continue what I was doing before it crashed?

Did you set the file as Active when you loaded it back up? I can't recall if it brings up the in-use, but it might do since a non-active Mod would be in use. You can double-check by opening the 'Data' tab, clicking your mod and seeing if it says 'Active' next to it; if not, just reload, and when you select your mod click 'Set as Active'