If your faucet has come loose from the countertop is can be because it has not been installed correctly. The faucet not only needs to be tightened to the countertop with the nut but also the two screws on the fastening gasket need to be tightened to put back pressure on the plate to lock in the faucet. Below is a diagram of the faucet and a video on how to make sure your faucet is tight to the countertop.
https://youtu.be/4wwAQJMdKTg