Headphones are a valid check. Make sure you plug into the speaker out port on the card, also check the headphone jack on the front of the PC or CD Drive (if equipped).
If you still don't have sound:
1. open the case and re-seat the card if it is a separate card.
2. uninstall the device from device manager, then re-install the drivers. Make sure you have the drivers first.
Thermal expansion, vibration, and nudges of plugs up against wall may lead to the card becoming slightly dislodged.
I've had sound blaster drivers just go bad from time to time. Great cards, but sometimes they have a buggy driver release, or if you downloaded a windows update driver, they frequently suck. Re-installing the drivers usually worked for me.