Brenda,
The products availiable over the counter won’t work well enough to remove a serious blockage. The products that plumbers can get are dangerous to your health, and can destroy traps and drains under some circumstances. You can try a hand snake if there is a removable strainer, but they are tough to work through a tub trap. If the trap is accessable through a wall opening, there may be a cleaning plug at the bottom of the trap. Or the trap can be disassembled and cleaned. If the clog is not in the trap, the drain line will need to be snaked. If all of this sounds strange or scary, call a plumber. PS. If you have been dumping chemicals in the drain, DON’T open the trap unless you are sure they have rinsed through. You can get chemical burns on your skin or eyes!