H and welcome to Android Forums!
If I woke up in your shoes, I would immediately connect the device to a PC and mount it as USB storage to see if the pictures can be located using Windows Explorer. Navigate to DCIM or Pictures or search the phone for
*.jpg and see if they are still on the phone. If they are, I would suggest copying them to a folder on the PC for safekeeping until the Gallery can get fixed.
If you cannot refresh the Gallery and see your photos, then something in the phone's hidden system files may have corrupted it. A factory reset may be the only way to get it in order again: but the phone will lose all user data, apps and settings. It will be in out-of-the-box condition. But by moving your critical photos, videos and music to the PC first, you can always copy them back.
Your apps are saved in the Google account, and should automatically restore when you log into Play... but your data will be lost
To do a factory reset, turn the phone off. Then hold the Volume DOWN button and Power button at the same time: this will get you to Recovery. Use the Volume Down button to navigate to "Wipe data / factory reset" and the power button to select it.
You will then be prompted to verify your choice. Volume down to the appropriate line and use the power button to select it. Your phone will wipe user data and the caches, and re-write the entire system partition with the stock operating system, then when complete, select "Reboot System Now" to restart the phone.
You will be taken through the entire setup process again, just like when the phone was new.
The only other thing I can think to do BEFORE a reset is to go to Settings --> Apps --> scroll over to ALL and look for Gallery. If you are able to see it and clear cache & data, try that before the reset and reboot the phone.
BACK UP YOUR PHOTOS BEFORE DOING THIS!
I hope this helps. Good luck. If that doesn't fix it, the OS may be corrupted and will need to be reflashed at your provider's facility.
It's great to have you here