Looking on the internet deeply has found these results:

What happened? is the most popular phrase on the web.

what happen? 114,900,000 results on the web	What happened? 1221,000,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	What happened? Riggs and Dubrow They killed What happened? What happened? (clears throat) They An executive meeting Everyone kept asking, "What happened?"

what happen?

114,900,000 results on the web

What happened?

1221,000,000 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.