The simplest, and most expedient way to open a desk drawer that has something jamming it, is to remove any surrounding drawers, according to SFGate. Once that is done, the item causing the jam should be accessible. Then it is simply a matter of removing the item and opening the drawer.
Sometimes there are not any surrounding drawers to remove, or the drawers are individually compartmentalized, so that removing them does not allow access to other drawers. In this case, a long, thin, sturdy object such as a metal ruler or butter knife comes in handy. The desk owner should slide it down each side of the drawer, including top and bottom, to see if the object responsible for the jam can be dislodged.
If this does not work, and the desk is small, the desk owner should try tipping it to the side, or tipping it over entirely to see if the item becomes dislodged. Sometimes gently shaking the desk can get the job done. Another option, if there are screws that are accessible, is to remove the face of the drawer in order to reach whatever is stuck inside of it. If all else fails, and it is relatively unlikely that the item doing the jamming is valuable, there is the brute force method of just grabbing the drawer and pulling as hard as possible.