Blender randomly not working

I'm currently on a mac ( with blender all updated) and I was wondering if anyone has the same problems as me. Sometimes from 30 minutes to 2 hours, blender starts to not function properly. Everything looks speedy, and there's no lag. It's just that I can't edit anything. I can't move, resize, or even undo anything. Does anyone else have this problem? And if you do, how did you fix it? An answer would be greatly appreciated! (Also I'm fairly new to this subreddit -I believe - and I'm surprised how everyone is so nice and supportive!)