Sometimes screwgate karabiners get stuck and they can be stubborn little buggers to release.
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. If 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.
About the Author
Alex Palmer is the co-owner and founder of Cold Mountain Kit and started climbing back in 1989. He's climbed extensively throughout the UK, French Alps and has been as far afield as Yosemite, Northern Patagonia and South Africa. A total trad climbing snob.