I have tried to import into LR 5.6 pictures taken with my Nikon DF (raw format). The import process has started but was interrupted. I had to reboot the computer. Since then, when I launch LR5.6, I get a message saying that LR has encountered an unknown problem, is trying to resolve it and tells me to close LR and reopen it, that it could eventually be fixed...LR 5.6 cannot be started anymore!
Go to the folder in which your catalog is located (default location C:/Pictures/Lightroom) and look for a file called (your catalog).lrcat.lock. The lock file is created when LR is open and deleted when you close the program. It prevents more than one catalog from being open at the same time, but sometimes when there is a system crash it is not deleted and therefore LR can't open a catalog. If you find it, delete it, but be sure it is the lock file you are deleting and not the catalog.lrcat file.