Hey guys,
So alex not much to say, I would say you first hold the power button for a while 20 seconds for example, make sure the battery is properly plugged, take the battery out wait some seconds and put it back again, and insert charger on, if this fails then you will have to open your device again :/. Try holding as well power and volume up, for a while, looks to me either is a battery problem, or a bad connection inside.
If you open make sure every single connection is properly connected, and make a clean of every connector specially the power ones use a 99% alcohol, and a soft tooth brush and some cotton buds, you would be surprised how the fat of our fingers can block connections, your next door pharmacist will probably have it (I live in Germany and its pretty fun, every time I go to buy I have to convince them for 30m. That is for electrical purposes and not for partying). if you follow those above procedures, I would say you have good good chances that your phone is back to life including the sensors