First, hold the carabiner horizontal to your body, with the screwgate facing you and wrap your index finger around the screwgate and use it a little like a spanner.┬áIf that doesn’t work a gentle tap with another carabiner or even a rock nut will often release it. If it’s still stuck, carefully apply a little heat to it with a lighter and then try the index finger trick again (not enough heat so as you burn yourself). If none of these tricks work, then you can use the pliers of a Leatherman or similar, but be aware this may permanently damage the ‘biner.