It's probably due to the oil level being too low. You could just pop off the top cap from the stanchion and top up, but the oil that was in there will have gone somewhere so you might want to service all the o-rings and replace the wiper seals and add fresh oil, give the internals a nice clean and add some suspension lube to the bushings and moving parts. Will help give your forks a longer life and they will be nice and plush afterwards and will thank you for it.
If you opt for just a top up, it's difficult to know how much you'd need, but basically with the damper locked out, oil can't move, too much and it will impact the forks performance.