How do I turn on the phone if power button is broke?

My phones power button broke and I had downloaded an app to where I can unlock the screen without it but here recently the phone has been shutting off randomly before it broke and now its turned off with it broke and I can't get it to turn back on. If I plug it in to the charger it shows it charging and the Blu screen and I've held down the volume button and that test thing pops up and I don't know how to get off it.