When I’m pedalling on my BMX it has no resistance - it feels like a mountain bike in low gear. Does anyone know how to fix it?
I believe most bmx bikes use a single speed thread on freewheel. If the cranks, chain and rear sprocket turn independently of the wheel your freewheel broke.
Thread on freewheels are not expensive and are fairly easy to replace, but you need a special tool that engages with splines on the freewheel body to unthread them.
Park Tool has a good video showing you how to replace a freewheel. The freewheels in that video are all multi sprocket but the procedure is the same for single sprockets.
When buying a replacement you just have to select the right sprocket size. The thread sizes are standard
Your gearing ratio is too low. You should try using either a bigger chainring or a smaller free wheel. You can also learn a bit more about it here.