The latch on an oven door withstands a lot of wear and tear. The door is constantly being opened and closed and this can knock it out of alignment, preventing the latch from operating properly. The other problem with latches is that most people do not think to clean them when they clean the oven. This can lead to a build-up of grease and dirt that can prevent the latch from working properly. Fixing the latch isn't difficult.
Mix 2 tbsp of dish soap with 1 gallon of hot water in a bucket.
Clean the latching mechanism completely with the soapy water and a rag. Look closely for any dirt in the mechanism and try to dig it out with a screwdriver. Clean the door part of the mechanism and the part on the main oven body.
Try opening the door latch. If it does not work, unscrew the door from the hinges with a screwdriver. Lay the door flat on the floor.
Pull the hinges out with pliers until you see the hole at the top. Stick a long nail or screw through the hole to prevent the hinges from retracting.
Reinstall the door onto the hinges and screw them in place. Remove the nail and try the door again.
Contact the manufacturer of the oven and order a new door hinge if nothing else fixes the latch. The new latch will install in a few minutes, using only a screwdriver. Follow the instructions that come with the new latch to install it.