DIAGNOSIS Grime, concrete dust, or dried-up grease has gotten into the detent ball and spring mechanism under the ring, jamming it in place.
FIX Don't force it with pliers—you'll break the plastic ring. 1) Blow compressed air around the base of the ring where it meets the tool body. 2) Apply a penetrant like WD-40 or a silicone spray around that same seam. Let it sit for 10 minutes. 3) Try to gently work the ring back and forth. The goal is to let the lubricant seep in and dissolve the gunk holding the detent ball. 4) Once it moves, work it through all positions repeatedly, adding a bit more spray.
IF THAT DIDN'T WORK If it's truly seized, you may need to partially disassemble the gear case to clean the mechanism. The ring is connected to a cam inside. For the 2904-20, this requires removing the chuck and then the front gear housing. This is a repair for someone comfortable with tool teardowns, as you'll need to regrease gears during reassembly.
HEADS UP Once freed, keep it moving. Cycle through the modes every couple of weeks if you don't use them often. The most common cause is leaving the tool stored in one position (especially hammer mode) for months in a dirty environment, allowing everything to lock up.