[Darryl Ramm]

I'll suggest the most basic troubleshooting.

Where is the Mix knob set? For listening to Pro Tools output make sure it is set all the way to "PB" (playback) and the headphone volume is turned up. If that is not it...

Power off the interface
reboot the computer.
Once is is fully restarted power on the interface
Start Pro Tools.

What happens? If that fixes it, ... ...you need to check that the computer is properly setup/optimized to not sleep/suspend and you must never manually do so while the interface is connected or the driver is likely to hang, that is just the standard expected crappy behavior of Avid drivers.

Try removing all other USB peripherals you can from the computer and reboot and try the interface on several different USB2 (not USB 3) ports on your computer.

If that does not work, try trashing prefs and databases and repeat the above (instructions for trashing prefs etc. are under the "help us help you" link up top of this web page).

Also confirm the headphones still work by connecting them to something else.

Make sure the headphone volume is turned up on the interface.

And confirm the headphone output stage in the interface and headphones are working by connecting a microphone to the interface and using the interface's hardware monitoring (mix knob set all the way to "IN" for input) to listen to the mic input (completely independent of Pro Tools). If that works set the monitor know all the way back to "PB". If that does not work you likely have a hardware problem in the interface.