The idea is that the tip is clogged. And you want to allow it to flow again.
You may try to dilate the tip (and slim the ink) with heat to allow the ink to flow. Other answers have already pointed to heat based solutions, such as using a candle.
Another option for the heat idea is to use pen against glass very fast. Glass won't get a permanent mark, and it will be hard for the ballpoint pen to scratch it. The fast motion is meant to heat the point of the pen by friction.
You may also try to use gravity (leaving the pen in a verticall position) to try to force the ink to flow. Other options are to use centrifugal force (that is a virtual force, it exists), or - in the case of cheap rollerball pens - dismantle the pen and blow the ink to the tip from the opposite side - beware of excess flow, do it against something that can absorb the ink.
On the other hand the ink may be dry. If it is not totally dry, you may try to mix it.
What kind of ink is it? Ink is usally a suspension in water, oil or alcohol. Rollerball pen usually are water based, others may use oil. Alcohol is only common in markers.
Whatever it is... add that to the ink - or the tip, if you don't have access to the ink. Water (Saliva from licking has been also suggested), Mineral Oil (Johnson's Baby Oil will do) or Alcohol (Isopropyl or in its defect, Methanol), depending the case.