Several possibilities may be the cause of a leaking skylight.  The leak may not actually be a leak at all but the outcome of condensation forming on the inside of the glass of the skylight during cold weather causing what may appear as a leaking skylight.  Another cause of your leaking skylight could be by an unconnected roof leak above the skylight that is finding its way out through the ceiling opening surrounding the skylight.
Sourcing the Spot of the Leaking Skylight
If the leak seems to be coming from the skylight …

During this time of year with the large amounts of snow on the rooftops many people who have skylights installed begin to have problems with a leaking skylight.
Possible Reasons for a Leaking Skylight

There are several possibilities for a leaking skylight and sometimes it may not even a leak. Leaking skylights can sometimes be due to condensation forming on the inside of the skylight during cold weather which then drips into the home. The leaking skylight could also be caused by an unrelated roof leak above the skylight that is …