Can't hear any sound for no apparent reason

Hi guys,
My computer audio was working fine yesterday. I left it on throughout the night but now all of a sudden, I can't hear anything from it.

I've checked a few common problems that may occur but to no avail. I've tried checking the:
- Sound level sliders and mute settings (placed all on maximum and unmuted)
- Soundcard driver (updated to the latest one)
- Speaker jack on another device (works fine)
- Speaker volume (placed it on mid to maximum)
- Ran some songs on a few media players i.e. WMP, VLC, mpc-hc (max volume)
- Played some youtube videos (max volume)

I've even restarted my computer and ran the audio playback troubleshooter with no satisfactory result. I'm pretty sure the problem is not as severe as a faulty audio output socket but I can't seem to get my mind around it. Help, please

Have you tried a System Restore to 2 or 3 days ago, before this problem began?

Thanks for the quick feedback

I just restored my system to 7th May but a dialog box popped up mentioning the restore wasn't successful. Despite that, the sound have returned without any manual changes. I'm not sure what exactly happened.