A sulker is easy to spot. They're the people who won’t talk to you when they're annoyed. Yes it's childish, and yes it's unprofessional, but some people, surprisingly, still do this.
Some people just geniunely can't find the words to tell you how they're feeling, or are too emotional to talk about it. Other people do it just to make you feel bad.
How to deal with a sulker
Dealing with a sulker can be really tricky at work, especially if you're working on a project together. So what do you do?
First, try to be understanding. Show that you're willing to listen when they're ready to talk.
If that doesn't work, tell them that their silence treatment actually makes you feel a bit uncomfortable, and that you want to sort it out. Try persuading them to talk.
As tempting as it might be, don't bother giving them the silent treatment back. You'll just end playing a long, long game of who will speak first.