If the gel (liquid) inside the body of the electrode is “low” (i.e., less than half the length of the body of the electrode), the sensor may still work properly. However, it may need to be calibrated each time before use (How do I calibrate a sensor in Logger Pro?)). It’s a sign of age or improper storage. If there is no gel, the probe is possibly dead and cannot be repaired. Most likely the bulb at the tip is broken or at least cracked (The glass bulb of my pH Sensor is broken. Can it be repaired?).
A quick test of the sensor is to take readings in fresh buffers, NOT distilled water! If no buffers are available, try it in vinegar (~ pH 2.5-3.5) and ammonia (~ pH 10.5-11.5). If readings do not change when the sensor is placed in different solutions, the sensor is possibly defective or damaged.
Note that this is related to the liquid (gel) level INSIDE the body of the probe itself, not the small plastic electrode storage bottle that contains a solution for soaking the tip of the sensor. If the electrode storage bottle is low or empty, then it should be refilled at least halfway with a solution that is a mixture of pH 4 buffer and KCl (the solution is prepared by adding 10 grams of solid KCl to 100 mL of pH 4 buffer solution). See Where do I get pH or ORP Storage Solution? (Buffer Storage Solution)