What should I do?

On my way home from work I stopped at a drive thru window and suddenly a light on my dashboard that said brake came on and then my car just shut off. I had to push it out of the way and after a minute or two it started back up and I was able to race down the street to where I live. In my parking lot I noticed that there is an air hissing sound when I press the brake that I didn't notice before. I want to take it to a mechanic but I'm a little shaken up now and terrified of it shutting off again, possibly on the middle of the highway. Anyone have any idea of what could be the problem? Also, I don't know if this is relevant or not but for the past month there's been a grinding sound when I accelerate but it usually goes away after a little while. Lately, it hasn't gone away.