Space is vast, so when some piece of human technology goes missing outside of Earth's atmosphere it's very difficult to find it again. And that task gets especially hard when the object is very small and isn't emitting any kind of signal.
In October 2008, the Indian Space Research Organisation (ISRO) launched its first lunar probe called Chandrayaan-1. It successfully entered a lunar orbit in November 2008, but in August 2009 things started to go wrong. Technical problems began to surface including sensors failing and thermal shielding not working effectively. On August 29, 2009 contact was lost.
Chandrayaan-1 remained lost until now, some eight years later, and ISRO has NASA to thank for finding it again. NASA discovered Chandrayaan-1 maintaining a lunar orbit 124 miles above the surface using a new technological application of interplanetary radar developed by NASA's Jet Propulsion Laboratory.
NASA actually located two spacecraft using this new technique and ground-based radar. The first was the agency's own Lunar Reconnaissance Orbiter (LRO), which was easy to find as the mission's navigators were on hand with precise orbit data. For Chandrayaan-1 however, it was a much more difficult task.