One day, it's all over. An actor leaves, and the whole show loses its focus. The writers change and it's no longer funny or dramatic. Some genius decides to boost ratings by adding a character only they think is amusing or interesting. There are lots of reasons why it happens, but it never fails to disappoint.
Some shows recognize when the party's over and bow out gracefully.
Others get lost in rescheduling shuffles. Some die a slow, painful death as executives try one last ditch effort after another to save it. The recent past has really hurt because I lost fun shows with great, non-bimbo women characters. None of these programs were ever masterpieces, but there were a lot of good laughs and occasionally some nicely done women's issues. The stories were cute and the supporting characters were zany enough to generate a lot of fun screwball stuff.