First of all, an occasional build-up of condensation is normal and often the result of fluctuating humidity in the home. Usually, it’s nothing to worry about. If you’re using a humidifier, try adjusting the levels. If the humidity is being generated naturally, try placing a dehumidifier nearby. Also, remove any plants and firewood from the area, as they can release a surprising volume of moisture into the air.
Do you see moisture in between the panes of glass that make up the window? If so, that means the seal has failed and moisture has crept in. Double and triple pane windows often contain a gas (argon, for example) that boosts the insulating qualities of the window. When the seal fails, the gas disappears, making the glass colder and often allowing condensation to creep in. Eventually, you’ll want to get it replaced.
If you see moisture build-up anywhere on the frame of the window, particularly at the joints, that could be a sign of water leaking through. That’s an issue you should get checked out immediately by a window contractor.