What's the matter? what happened? What's wrong?
is there any difference between these question forms? I mean can I use them in place of each other ?
What's the matter?
what happened?
What's wrong?

Question

What's the matter? what happened?  What's wrong?
is there any difference between these question forms? I mean can I use them in place of each other ?
What's the matter?
what happened? 
What's wrong?

Answer

You could use all of these phrases if you felt something was wrong and you wanted to know the cause.
The only difference is the "What happened?" could also be used if something good happened - the other two phrases could not.

"Janet looks much slimmer.  What happened?  Did she start working out or go on a diet?"

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