Formal:
* എന്തായി? (enthaayi?) - This is the most common and formal way to ask "What happened?".
* എന്തു സംഭവിച്ചു? (entu sambhavichu?) - This is a slightly more formal way to ask, literally meaning "What happened?".
* എന്ത് സംഭവം? (ent sambhavam?) - This translates to "What event?".
Informal:
* എന്താ? (entha?) - This is a very informal way to ask "What happened?".
* എന്തായി പോയി? (enthaayi poyi?) - This is a slightly more informal way to ask, literally meaning "What happened?".
For specific situations:
* എന്താണ് സംഭവിച്ചത്? (enthaanu sambhavichath?) - This is used to ask about something specific that happened, literally meaning "What happened?".
* എന്തുകൊണ്ട് സംഭവിച്ചു? (entukond sambhavichu?) - This translates to "Why did it happen?".
Other options:
* എന്താ പറ്റിയത്? (entha pattiyat?) - This translates to "What happened?".
* എന്താണ് പറ്റിയത്? (enthaanu pattiyat?) - This is a more formal way to ask "What happened?".
The choice of phrase will depend on the context and the relationship between the speakers.