Minecraft Connection Problems

A while ago, I let my friend sign in to my Minecraft account on his lap top for about a day and he claims to have signed out. I for one don't know if he really did because when I tried to play Minecraft again, it wouldn't let me connect to any servers. When I started Minecraft back up, it made me sign back in, which I understood why but I don't understand why I can't play on any servers. I already tried reinstalling Minecraft, shutting my computer down and even restarting my wifi, nothing seems to work and I still seem to be getting the same message when I attempt to play on a server, which is "java.lang.NullPointerException: group" Any help would be appreciated, thanks!

You should check your account on Mojang website and try to login there. Probably good idea is to change the password, just for being sure. If your password doesn't work, you as true owner of the account should not have any problem to reset it by using security questions.