It's rare for a bathtub to suddenly become stopped up. A clog in the tub usually builds up over a period of several weeks, with the tub draining more and more slowly each day. We've all seen this happen.
As with a sink clog, start with the plunger. First, unscrew the screen from the tub drain and use a bent wire to fish out any hair and soap scum. If there's a pop-up drain on the tub, raise the lever to the open position, then grab the stopper and pull it from the drain hole. Clean it of all hair and soap. This will often take care of things.
If not, cover the holes on the underside of the overflow plate with a wet rag and start plunging. If that doesn't clear the clog, use the cable auger.