I just called someone less than 5 minutes ago, sat my phone down, and now it will not turn on. I charged it, took the back off, put it back on, and nothing worked.
Take the battery out, put it back again and boot in recovery mode. To do that, press the Volume Down key and Power button simultaneously until recovery boots. Then there should be an option to reboot. Select this option. Your phone should turn on. If not take it to a service center or wipe data/factory reset from the recovery menu.