My bathtub water has stopped draining. Now when we use the bathroom sink, the sink water backs up into the bathtub. What do you think is the problem??

Tubs clog with hair very quickly. Use a liquid drain cleaner, or lye granules with water, or a snake, or take the trap apart below. And buy a $2 plastic hair trap, either an 'innie' or an 'outie.'

Both fixtures eventually drain to the same line which is clogged. This is after the tub trap so a Zip it Zip-It Clean won't do the trick. If there are no clean outs below the tub you can remove the over flow plate or stopper trigger and rod down there. How to Clean Out a Tub Drain | Danny Lipford

Draino always does the trick for me!