Adjust Levels Sliders Not Working

So this is really frustrating me. I have a macbook pro and when I go to adjust levels on a layer by command L and move the sliders - nothing happens. The image is exactly the same no matter how drastic the sliders move and I have no idea why. If I create a levels adjustment layer and move the sliders, it works fine. But adjusting the levels on the image directly doesn't work. I updated to photoshop 2017, I restarted my computer, I quit and reopened photoshop multiple times and nothing works. All the other tools work fine, but adjusting levels which unfortunately is my most used tool doesn't work. I don't want to restore photoshop to the factory settings because then won't I lose all my custom brushes and fonts? Has anyone else reported this problem?

Thanks in advance xx