I had the same problem.
What happened is last night i was about to sleep i checked my phone and any of the buttons aren't working, of course i went to search how to fix it and most of the suggestions are pressing the lock & home button at the same time for 10-15 seconds but mine didn't work considering i have a broken lock button for months now. and I went to search again and see i wasn't the only one so i felt relieved a bit.
I've read everyone's suggestions here but I did one strategy that worked 100%. What you should do is completely drain your battery (I left mine to drain for 11 hours) and after that you should charge it in the computer and wait. If the low battery sign turned on then u should wait for the apple logo then ur saved.