The pins and springs in any lock can seize up not allowing the key to go in. 1 small squirt of wd40 will do the trick and you only need to do this once then insert the key and move it back and forth a few times, this will help the wd40 get into the right places and free the pins. If too much wd40 is put in you run the risk of dust etc getting into the keyhole and mixing with the wd40 which may cause problems in the future. Always use wd40 dry lubricant which forms a film round the pins and does not collect dust

Buy WD-40 and spray the lubricant in to the keyhole, wait 20 sec and apply one more time. Then try to put your key, open and close lock few times, then spray lubricant in to the keyhole one more time. You can try to repeat this several times, and if it does not work call locksmith.

what kind of lock is it ? than I can tell u the trouble

Determin if it is a mortice lock or a cylinder type. If it’s a cylinder lock WD40 will lubricate the pins to allow the key to enter and turn, if it’s a 5 lever kitemarked mortice lock I suggest the bolt thrower centre I gotta spring has weakened so the cam is preventing the keys entry. The cam needs to be recentred using a thin electrical screwdriver then you can use your key to open the door. Once open call a locksmith to resolve the problem permanently.
Be lucky, Steve

The W in WD-40 stands for water, when you put water into metal it causes rust. A better solution is to get a bottle of lithium grease and put that in instead. Does exactly the same thing except it doesn’t cause rust to form inside the lock. Hope this helps.

This is an easy fix, all you need is a small can of WD40 which normally comes with a small red straw. Attach the straw then spray a couple of times in the key way. Next you can move the key in and out to see if this does the trick.