You can use the phrase "___ has lost it" to talk about someone who used to be really good, but isn't as good any more. This phrase is useful for talking about people like:
When you use this phrase, "it" means the person's skill, talent, or creativity.
There's also a completely different meaning of "___ has lost it". It can mean that someone has gone crazy:
A: What's wrong with him lately?
B: I don't know, but he's completely lost it.