Unscrew the upper or lower portion of the pen -- it can vary depending on the exact type -- and remove the cartridge. Don't lose any of the springs or other components that may fall out and remember where each one goes.
Inspect the cartridge to see if it still contains ink and reassemble the pen if this is so. These first two steps are only needed if this pen has an opaque shell with a clear cartridge.
Tap the pen's tip on a hard glass surface and then run the tip over the surface multiple times quickly to try to free up a stuck ballpoint.
Heat the tip of the pen with a cigarette lighter for a couple of seconds to melt any ink that may have dried within the tip.
Suck on the pen's tip with your mouth to try and remove any air pockets that may have built up. Be careful of any ink that may come into your mouth; spit it out and wash out your mouth immediately if this happens.
Apply a small amount of window cleaner to your finger while wearing a rubber glove and then rub the cleaning solution into the tip of the pen to try and moisten the tip.
Install a new cartridge in the pen if you have determined the current cartridge is completely empty and/or dry. The exact type of cartridge you need can vary on your particular pen.