When a spacecraft orbits around the Moon, as SMART-1 does, it is doomed by the law of gravity. Tugs from the Sun, the Earth, and irregularities in the Moon itself, all disturb its orbit. Sooner or later, any lunar orbiter will impact the Moon surface unless it has very big amounts of fuel left to be re-boosted and escape the lunar gravity.
By the time SMART-1 had settled into its operational orbit around the Moon in March 2005 its exeprimental ion engine had only 7 kilograms of propellant left (bottled xenon gas) out of the 84 kilograms available at launch. This fuel allowed orbital boosts, but was not sufficient for a Moon escape.