Lateral file cabinets are file cabinets that are much wider than they are tall. These units may only have two or three drawers, but these drawers may be four to five feet in length with some models. Lateral cabinets aren't top heavy, and they can be stacked on top of each other to utilize more space. However, these file cabinets can jam just as easily as their more upright cousins. Getting the drawers unjammed is a very similar process though.
Check the lock to make sure that the lateral file cabinet isn't locked. Attempting to open a locked drawer can give the impression that it's jammed, but if you have the key then the drawer's easily opened.
Take out the drawer above the jammed lateral file cabinet drawer. If you can see down into the jammed drawer then you may be able to make out the problem. For instance, if there is a file sticking up too high then it might be blocking the drawer from opening. If you can fix the jam from above then replace the drawer when you've finished unjamming the file cabinet drawer.
Push and pull on the jammed drawer gently. If the drawer slides smoothly then it's one track. If the drawer doesn't slide smoothly then it may have fallen off the track that lets it open. Use a prybar to lift the drawer up, and then slid all of the tracks into place if you can. Some file cabinets have a makeup that won't allow you to pry at them, but others are easier to slip tools into.