Fatal error: Call to undefined function

Provide some more details. Fresh installation? Upgrade from version x?

To fix the issue for others I need to know which version was installed before you upgraded to 3.6.

Not sure if you still have this problem, but this is what happened to me... Maybe it will help someone else in the future...

I upgraded ThemeKey from 6.x-3.6 to 6.x-4.0 in early February 2012 with no problems or issues.

This morning (March 9) I did some module updates and afterwards received the same fatal error mentioned above on all pages except admin pages. Unfortunately, the backup I did right before the update had the same error as well. I didn't notice the error before the backup because admin pages worked fine and I didn't have a need to visit the "normal" pages at that moment.

After rolling back the working/live site, I moved onto my development site and was able to recreate the error by restoring my backup.

I visited /admin/settings/themekey and BAM! All the pages worked again; you don't even have to press 'Save configuration'.

I'm not sure why it happened, but I've tried this 'fix' 3 times and it worked every time on the development site.

The live site is working ATM so if it happens the next attempt to update and this 'fix' doesn't work, I'll follow up.

Thanks for sharing these details.

maybe a similar issue:
#1600844: Themekey fatals on External Pages with Bootstrap