Okay from what I can tell this seems to be a settings issue. Go to windows>seetings/preferences>preferences then choose edit/reset default settings. then see if the problem persists.

BE VERY MINDFUL OF ANY EXTRA/INADVERTANT KEYSTROKES AS YOU ARE MODELING. It is very easy to turn on and off things by accident in maya that will drive you literally INSANE! As an example just press the b-key and you are in soft select mode. People just learning turn this on all the time. I think I see this question come up at least once every couple months.