In minecraft PC I was on survival. I went on peaceful then switched to normal but no monsters are spawning. Why?
Monsters only spawn in areas of darkness (eg. a cave or at night-time) and will not spawn within a certain number of blocks of you. Try walking around for a bit and see if they start appearing then. remember they won't spawn instantly, you have to wait for a bit.
If everything above is correct, and they still won't spawn, try closing and reopening the game, or relogging it. if none of these things work it could be glitch, or a corrupted world, in which case you'll either have o try to fix it, or start a new world.