A tub stopper can be very useful if you want to fill up your tub, but if it fails, there could be problems. Many older tubs have a stopper that is attached to a chain inside of a channel in your tub. You operate it by moving a lever up and down, and it will lift up to open up for water flow and go down to close the water flow off.
It happens sometimes though that the chain gets disconnected and cuts off water flow permanently. This could be a big problem, but it is not very hard to fix.
By removing the lever, you should get access to the channel where the stopper is located. By using a wire hanger, you can fish it up out of the channel and reattach the chain. There might be a need to adjust the length of the chain to make sure that it functions properly, or if you don’t want to have a stopper anymore, you could just remove it.