You can declare your ball unplayable in a bunker and you options are the same as elsewhere in the general area ie under penalty of one stroke you can

1. Go back from where you played into the bunker, stroke and distance
2. Drop a ball within 2 club lengths of the ball, no nearer the hole, inside the bunker and
3. Drop a ball back on the (imaginary) line from the flagstick to where the ball is at rest in the bunker, going back as far as you like but still in the bunker.

If you choose options 2 or 3 you may be very restricted in where you can drop the ball inside the bunker. When the rules were revised in 2019 an additional option was introduced and you may now take the ball out of the bunker altogether for two penalty strokes so you may drop a ball back on the line behind the bunker on line with where the ball is at rest and the flagstick. It is well worth knowing this option as it is in addition to the stroke and distance option which will only cost you one penalty stroke but you may not want to go all the way back to where you played the ball into the bunker. Rule 19.3