In 2009, Jay Leno left The Tonight Show and was replaced by Conan O'Brien. Later that year, Jay launched a new NBC show that aired in prime time. The show did not do well with ratings.
NBC then proposed an idea in which Leno would host a half hour problem after the local news, and Conan would be pushed to midnight instead of letting his program start earlier and when 'The Tonight Show' usually did. Conan did not like the idea, felt he was being cheated and left The Tonight Show and the NBC network altogether after only a few months on air. After this, Jay returned to hosting The Tonight Show.
So basically, Jay gave Conan The Tonight Show, but after his new show failed miserably, he took back The Tonight Show and left Conan with nothing.