My headlights just randomly stopped working but yet the running lights still work, what's wrong with it?

I was driving and my headlights just all of a sudden stopped working, the running lights work but not the regular headlight or even the brights. I have replaced the bulbs, I'm not sure about the fuses, I have never dealt with those before I wouldn't know which one is which. I was told its either the fuse or the ground wiring or it could be something else. I am very confused but I would like to try and fix it. Do you have any idea what it could be and how I could test and see what is wrong with it? I would greatly appreciate any advice.