SMSS will start for one user, but not another?
Perhaps something in their documents\user data profile got corrupted. If you've fixed the space issue, then I suggest running a repair on the application (or installing a newer version - you can manage SQL Server 2012 with SSMS 2014 or newer, and it's free - installation instructions in this answer). However it may still end up being hosed because of that user's profile - if so, as a next step, you could back up their files and re-create their profile (though I do concede that this will force you to lose a lot of their other settings etc).