Open the drawer as far as you can, even if it is just a crack, and use a flashlight to look inside. See if you can see an item wedged behind the upper lip of the drawer. Insert a long, flat tool, such as a flat-head screwdriver or a butter knife, behind the lip and push the item down back into the drawer. Slide the drawer open. If you only get the drawer partway open, remove as many items as you can from the front to give the items in the back more room to settle. This will allow the stuck item to fall into a better position.
Slide a flat, wide tool such as a putty knife around the edge of the drawer where the drawer and cabinet body meet. Work the tool all the way around to break any seal that has formed. A freshly painted drawer, high humidity or a spill can cause the drawer to seal shut and not open. As the seal breaks, wiggle the drawer as you pull it open to help it loosen.
Remove the drawer below it and reach up behind the drawer to get a better grasp on it. Put your other hand on the front of the drawer and push, wiggle or shake the drawer until it comes loose. If the drawer isn't sitting correctly on its tracks or it is old and the wood doesn't slide well, you can get enough leverage in this position to move it back on track and open it.