Why won't my iPhone 4 turn sideways?

When I'm watching videos my iPhone 4 won't let me turn it sideway so please help me

You have your iPhone on lock to it only faces vertically. If there's a picture of a lock with something like it then follow my instructions. 1 go to settings 2 go to general 3 find accessibility 4 look for assisitiveTouch 5 turn it on 6 click on the square thing that appears on your screen after you turn it on 7 click "device" 8 press "rotate" and choose left, right, upside down, etc HOPE THIS HELPS