If you have not yet, plug into the AC power so your battery has plenty of charge.
Do a Soft Reset by holding the Power + Volume Down buttons until the phone vibrates and reboots (about ten seconds).
If it boots back to the Lightning Bolt and Gear screen you will have to do a Hard Reset which causes loss of data.
Turn off the phone.
Hold the Volume Down button and tap the power button.
The phone should boot to an Exclamation Point screen - release the Volume Down button
Tap in order: Volume Up, Volume Down, Power, Volume Down
The phone will flash the OS version stored in memory and you will have to do your setup again.