Barn Owls are completely non aggressive when cornered. I promise you, the most it will do, is lie on its back, and if you pick it up, it will not even try to peck or scratch you. Do not use a net, they look a lot more robust than they actually are. If you open the door, and the Owl does not fly out, it is probably weak from hunger. (They need to eat every day) If it is too weak to fly, do not try to feed it meat, wrap it gently in a soft towel, lie it on its back, and try to drop a solution of glucose into its beak. This will give it the strength to eat some strips of liver,or chicken breast. You should ring the RSPB, and find out what it's Adult body weight should be, and then weigh it, (it won't be a juvenile Bird at this time of year. If it is more than 1 fifth under its normal body weight , you might have to sit up all night feeding it glucose, just to get it to survive. They are surprisingly delicate Birds, and weigh only about as much as a pigeon, and are very susceptible to
poisons, and in the wild, feed almost exclusively on Voles. If it doesn't fly away, straight away, try and get in touch with someone who keeps Owls, as it will need re-habilitation if it is to survive. It will not readily take to any new diet, as it is used to freshly killed voles.It will need to be successfully fed day old dead chicks for a few days, (unless you have any live voles) and this is why they are so vulnerable, and need the glucose.
Its claws and beak look formidable, but it will not attack or fight you at all. (It knows it can't do anything bigger than a baby rabbit) and it will go limp.
If it doesn't fly away, and you don't get any of this special treatment to it, it will be dead by tomorrow night.