Was testing a batch file to add a user to a localdb instance. Assumed that my user as admin on the machine wouldn't have an issue inserting myself back…. didn't think that one through too carefully. Executing any type of SQLCMD against it denied me. SSMS denied me. No SA had been setup on it, so I couldn't login as SA either. Looked for various solutions, and ended up uninstalling and reinstalling (localdb)v11.0 so that I'd stop having myself denied permissions.
This however, didn't fix my issue. The solution that ended up working from me came from dba.stackstackexchange.
I ended up deleting everything in the v11.0 Instances folder and then issuing the following command
sqllocaldb.exe c v11.0
Resulting in message:
LocalDB instance "v11.0" created with version 11.0.3000.0.
Success! This resulted in the instance being created successfully, and then I was able to login with SSMS. Apparently today was my day for learning some localdb permissions issues. What a blast….. Could have avoided this if I had simply used a test login, or had setup the SA with a proper password for logging in.