Last night we released the Nexus Mod Manager version 0.15.0 for general download to fix the issues that Bethesda and Valve presented all the mod managers out there with the changes to the way load orders are handled. Sadly Bethesda didn't give us nearly enough time to ensure we could solve the issues they've presented us (and I'm sure they'd argue they don't have to), so I know that wrinklyninja (of BOSS) has worked at least 100+ hours in the last week alone on getting the BOSS API to a usable state for kaburke (NMM) and Lojack (Wrye) to equally spend a lot of time updating the respective mod managers.
Unfortunately version 0.15.0 of NMM has a program breaking bug in it at the moment that is going to need to be hotfixed as soon as possible. Lots and lots of people have reported this issue so I think it's safe to assume that almost everyone has the same problem! There is a fix for this right now, however, if you are willing to do a little folder copy and pasting. Alternatively you can wait for a hotfix, that will hopefully come today, or roll-back to version 0.14.2
The fix is very simple. Navigate to where you have NMM installed (the default location is C:Program FilesNexus Mod Manager), copy the "data" folder from inside the Nexus Mod Manager directory, go back one folder (to C:Program Files) and paste the data folder inside the Program Files directory. NMM should now start, load, and work. If it doesn't then I'm afraid you have a different problem, and one that has likely been in NMM since before this version.
When a hotfix gets released for version 0.15.0 you'll be safe to remove the copied data folder from your Program Files directory.
Many apologies for the program breaking bug, but everyone involved has been scrambling and putting in ludicrous hours (especially wrinklyninja, who deserves your thanks) to get this problem sorted for you as quickly as humanly possible.