Checking gas valves and your tank level will ensure you have the correct levels to pour beer.
As part of changing the gas, you or your gas distributor would have had to shut off the gas line, including at the gas manifold and regulator lines. Ensure that all of the shut off valves are completely parallel to the line (not angled) to allow the gas to flow.
For a video with additional steps on changing a gas tank, click here.
After checking your valves, make sure your gas tank gauge level is in the green. Turning your tank's knob to the open position.