No worries. The tank valve has what is referred to as a dip tube (some call it a snorkel tube)The original reasoning behind the tube was that if there was water in your cylinder and you were in a head first descent the tube would act like a snorkel to keep the water from entering your regulator. These are screwed in and occasionally come loose and drop to the bottom of the cylinder. your cylinder is still usable. When you take it in for a refill ask the dive store to screw it back in. You can do it yourself but if a tank comes into a dive store totally empty it is considered out of visual inspection (no matter when the tank is actually due) This is common practice. If the dive store did the last visual inspection on the cylinder they would probably let you slide because it is a simple fix and they would have a record of the last visual. It helps to bring the cylinder pressure down to about 100 psi. When draining a cylinder the air is bled out slowly to avoid condensation from forming inside the cylinder. It is especially important if your cylinders are steel rather than aluminum. If the cylinder is full when you take it in, expect to have to leave it overnight as draining a full cylinder takes awhile. If you want to reduce the pressure yourself (not totally) open the valve of the cylinder slightly to where the lowest amount of "hissing" can be heard. But leave some air in it when you take it in.