2nd gear grinding and doesnt work sometimes

I know that this may sound dumb and im sorry for taking up your time, i just was wondering what is wrong with my car, I was driving down the road and when i shifted to second i heard a loud pounding noise from the trans and second gear was completly gone, i pulled over and checked it out and when i turned my car back on, second gear worked fine, and now its in and out, can anyone help me