You don't say whether this is an official update or whether you are rooted and using a custom ROM. I'll assume it's official and the phone is not rooted.
In which case, my first suggestion is to force a hard shutdown. Press the power button and hold it down. After 10 seconds or so the phone will shut down - you should see the capacitive buttons flash for a few seconds before this. If we're lucky this will clear something that's locked-up in the silicon. For some sensor problems I've seen booting into safe mode suggested, so give that a try at the same time: when the phone reboots and you see the splash screen, press and hold volume up (straight away). After it's fully booted test the sensor - your own apps will be disabled in safe mode, but reboot it again normally and they'll be back. Personally I'd turn off the "fast boot" option before doing any of this.
If nothing else works, and you are sure it was the update that caused it, you could try a factory reset. Make sure you have everything backed up first, but sometimes updates can leave some files in a mess, and if that's the cause a reset should clear it. But as restoring is a hassle, keep this until you run out of other options.
Good luck.