Why does my cake fall in the middle?

i baked this betty crocker box cake and added all the ingredients correctly and followed all of the directions. the box said bake for 31-38 Min's in the oven so i set it for that.. but i checked on it and after 17 Min's the top was brown and almost burnt.. so i did the toothpick check and nothing came out so it figured it was done.. but like 10 Min's later i went back to see if the cake cooled and it was sunk in the middle!!! what went wrong?!?!?!?