CO2 Leak

Hey all - first time posting after reading many a forum here. I kegged for the first time a few weeks ago and all went well for the maiden run. I've been enjoying the fruits of my labor for the last 3 weeks with no issues, but I went to poor myself a beer tonight to find my CO2 tank completely empty and no pressure in my keg. The regulator had been readng steady on what was left in the 5 lb. tank and it wasn't close to empty. Any idea what could have happened? It seems odd that it would have been fine for a few weeks and then all of a sudden spring a leak. I would love the collective wisdom here to help me figure out what I should be looking for.

Double check all your fittings. I found a slow leak in one of mine.

The pressure in the tank will stay pretty stable until all the liquid is gone.