Mid 2011 iMac Media Keys Not Working After El Capitan Update

[HelicalSlime]

After updating my Mid 2011 iMac 21.5 to El Capitan today, I've noticed that my media keys, specifically Back (F7), Play/ Pause (F8), and Forward (F9) are no longer working.

Most of the fixes I've found seem to involve a Google Music extension in Chrome which I don't have installed. I've tried looking through my iTunes preferences but didn't see anything that looked like it would fix the issue.

Any ideas what I could try to get my media keys working again?

EDIT:

After logging out and logging back in to fix the stuck Calendar date, my media keys are back to working like normal.

 

[redheeler]

Check if System Preferences > Keyboard > Use all F1, F2, etc. keys as standard function keys is enabled.

 

[HelicalSlime]

Check if System Preferences > Keyboard > Use all F1, F2, etc. keys as standard function keys is enabled.

That box wasn't checked but enabling it didn't do anything, it actually disabled my Volume Up/ Down keys.

Also, no idea if this is related, I just realized my Calander icon no longer displays the correct date- it appears stuck on March 5, the date I updated to El Capitan.