What is the difference between ‘what happened?’ and ‘what could have happened?
1) I couldn’t see Gina last month. Do you know what happened to her? (or Do you know what could have happened to her?)
2) I couldn’t see Gina last month. What happened to her? (or What could have happened to her?)
Well, ‘could’ indicates a possibility.
e.g. “I’m so glad you’re home safe, I couldn’t sleep thinking about what could have happened.” or “Why did you cross the road without looking? Did you even think about what could have happened?”.
Simply asking “What happened?” either means you know something happened and are asking for details, or you want to know if anything happened at all. e.g. “You look really sad. What happened?” but you’re not concerned about possible outcomes.