“Whatever happened to what's-his-name?”
You're talking to your ex-wife. She mentions that she's dating someone new. She had a boyfriend before, and you thought that they were still together. You don't remember his name, so this is how you ask about him.
Whatever happened to what's-his-name?
There are some things that you used to hear about, but now they're gone or not common. When you suddenly wonder about one of these things, say "Whatever happened to ___?"
Whatever happened to those leg warmers that girls used to wear in the '80s? Do they still make those?
You can ask this about people, too:
Whatever happened to Jacob? I haven't talked to him in forever!
When you can't remember the name of someone, you can call him or her "What's-his-name" or "What's-her-name":
It stars What's-his-name. You know, the guy from Titanic.
A: Where is it?
B: I gave it to What's-her-name.
B: Yeah, I think so.
In sentences like these, "What's-his-name" replaces the person's name.
This phrase can seem rude because it seems like you don't care much about what the person's name is. Or it can just seem like you're forgetful and relaxed.