Most likely some one has added a tire sealant to the tube and the core is not unscrewing. Are you turning it counterclockwise? If it is stuck and you cannot make it move then you will need to replace the stem by letting all the air out and braking down the tire bead. If it has a tube, replace it. If it is tubeless pull the old stem out and and put in the new one. If it continues to turn, the threads may be stripped and the stem replaced. If it has worked out a little, try using needle nose pliers to remove it.
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Most likely some one has added a tire sealant to the tube and the core is not unscrewing. Are you turning it counterclockwise? If it is stuck and you cannot make it move then you will need to replace the stem by letting all the air out and braking down the tire bead. If it has a tube, replace it. If it is tubeless pull the old stem out and and put in the new one.
If it continues to turn, the threads may be stripped and the stem replaced. If it has worked out a little, try using needle nose pliers to remove it.