Every now and then photographers encounter low light situations.  It can happen outdoors but mostly happens inside where the light is less abundant.  I have encountered this during a few of the weddings I have done throughout the years whether it be during the actual ceremony or the reception. 

The trick to over come this is a combination of things.  I usually start by opening up the ISO to at least 3200 and slow down my shutter speed to as slow as possible so as not to make the pictures blurry.  If you have the shutter speed open too slow normal hand shake will make the pictures blurry, so you have to find the fine line between blurry and sharp.  If you have time, unlike in a wedding when everything is moving at lightning speed, you can use a tripod to help with the blur.  Then you can slow the shutter speed down even lower to get the shot you want.  Likewise if you have the ISO open too much it will make your pictures grainy so you'll have to take some pictures and look at them to make sure they are how you want them to be.  One more thing you can try is to change the aperture on you lens to f/1.8 or f/1.4 to allow more light to come through. 

It's all about finding the perfect balance.  With practice and some trial and error you can get low light photography right.  And if all else fails, then ask someone if they wouldn't mind turning on some lights.