How do I turn on an iPhone back after turning it off using assistive touch? My lock button on the top is broken so I have no other way of turning on my iPhone.
By default, charging will turn the phone on. You can try charging the phone.
You can do as revolver said but if both your home button and lock button are not working anymore, you can goto Settings > General > Auto-Lock and turn it to never which will never dim your display and thus locking it will not turn off your screen.
Some problems with this however is it will drain battery power, probably isn't good for hardware and the screens always on, glowing jeans :)
If you really wanted to goto the hassle of doing so, you can turn your brightness way down and turning back up when in use which will be alot safer.