I wonder If I see a friend which looks injured, should I ask "Have you had an accident? " or "Did you have an accident?"

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All the above are good.  You could also ask, "Have you had an accident recently?"

Have you had an accident?

About an accident that happened in the past, no mention about the time frame when it happened and the incident has a connection to the present.

Did you have an accident? or Did you have an accident last year?

About an accident that happened in the past, mention about the time frame when it happened.

Have you had an accident? Did you have an accident?

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As said already, 'Have you had an accident?' is the correct option :) But just as a side note, I thought I'd just say 'Did you have an accident?' can also be used for when someone pees their pants. For example, a parent would ask their kid 'Did you have an accident?'

For that reason, I'd hardly ever say 'did you have an accident' even when talking about the distant past. Instead (if you're referring to a car crash), I'd be more likely to say 'Were you in an accident?' 

Just thought I'd add :)