You probably downloaded something from cydia that probably does not run well on your iPhone.

Go into safe mode and delete whatever you recently downloaded.

Maybe that's the issue

Simple to fix really, you're stuck in a bootloop like i was.

Turn of your phone and put it on vibrate, turn it on and hold the volume down or up button and keep hold of it till your phone vibrates. It will boot without cydia, tweaks etc.

Plug it in with itunes and just restore your iphone.