.bsp files are the compiled map files. These can only be played, not edited, and should be in your maps folder.
.vmf files are the uncompiled, editable map files. These are what you should be trying to open. You can find them in your mapsrc folder.
Edit: sorry, I didn't see that you already referred to the vmf.
This problem happened to me before. Did you reload your map from Hammer dialogue asking if you wanted to open it after it didn't close properly?