You'll have to disassemble it. There is probably a staple lodged in the "hammer" mechanism on the business end of the stapler.

Often, these staple are very difficult to remove -- a very small flat-tip screwdriver and needle-nose pliers usually work best.

After you get all the jammed staples out, lubricate all moving parts with something akin to 3-IN-1 machine oil. This should prevent future jams.

Charlie