Tap the ballpoint of the pen on a hard surface. You're doing this to attempt to jar the ball loose.
Take a piece of paper and scribble on it with the pen. Apply pressure to try and get the ball moving. This is the usual response whenever a ballpoint pen isn't working, and it will usually work.
If the pen still doesn't work, spray some window cleaner on your finger and rub the ballpoint with it. The window cleaner will usually clear up the dried ink and unstick the ball. Try scribbling the pen on a piece of paper again.
If the pen still won't write, try drawing some circles on glass (like a window). This can also help get the ball unstuck.