Diagnosis
Mineral deposits or scale from hard water can seize up the plastic bypass valve lever, especially if it's never been moved since installation.
How to Fix It
Turn off the cold water supply valve under your sink first. Then, gently try to work the lever back and forth. Applying a drop or two of a food-grade lubricant like silicone grease on the lever's pivot point can help. Don't force it with pliers—you'll snap the plastic.
If That Didn't Work
If it's completely frozen, you may need to manually shut off the water supply and relieve pressure by opening the filter faucet before attempting a filter change, bypassing the bypass lever entirely.
Heads Up
When you do get it moving, cycle it a few times a year to prevent it from seizing again. Always put it in the 'bypass' (horizontal) position before removing the filter housing.