99% of the time it's the euro cylinder. If it works one side but not the other it's not the key.
Just replace the cylinder. Remove and replace with correct size of previous one.
Don't start filing keys or it won't work in either side. And wd40 won't fix your problem either.
Leigh's locks

try WD 40 on it first. But if this happens with more than one Key its the lock, if another key works ok look at both keys if the one that works is smaller in anyway file the bad key to make it the same size,
sometimes if you move the key in or out slightly you can often find a way of making them work.
Kelvin

Hi

It's probably the key is worn,try another key.If that doesn't work if its a dead lock it could be the levers that are worn or some debris in the lock.Try giving it a spray with wd40.

Hope this helps

Steve

if its a upvc door with a euro cylinder probably the cylinder or the key is worn,
post the job on this website and get some quotes

domestic maintenance services

More info would be helpful but if it’s a wooden door you need to check your key, it needs to be symmetrical as each side uses different parts of the key, normally a prong is bent or broken off. Upvc doors it’s very unlikely to be the key if it works from one side, common problem is you’ve spun the cam by putting a key in one side when a key is inside... new cylinder required

If you're referring to a upvc door then the chances are that the internal half of the euro cylinder has something faulty with it. These are relatively cheap to replace although I would recommend ensuring it has at least a 1 star rating and best of all a 3 star rating. Quite easy to replace but if in any doubt call a locksmith.

If it’s a door with a Euro Cylinder it sound like the key is not catching the cam (the bit makes the bolts come out) this may because there’s a problem with the key or the cylinder (my money would be on the cylinder) Best course of action would be to replace the cylinder.

If it’s a wooden door with a sash or dead lock it will likely be a problem with the key, call your local locksmith

Your locks,strike plates and door will need an over-haul (ease and ajusting) to get back into alignment and working order.
I would be happy to do job if still needed.