"Что случилось?" и "что произошло?" - синонимы. Разница есть, но трудно уловимая. Их можно перевести на английский как "what occurred" или "what happened", как вы написали.

Thank you very much for explaining the derivation of the word "произошло"(from the verb "идти"), and "случилось" (from the noun "случай"). This can be useful for me. Thanks!

Okay, if we go your way, both words are Russian; the word "произошло" derives from the verb "идти" (to go), and "случилось" derives from the noun "случай" (an occasion). Произошло and случилось are synonyms too, but there is a very slight difference between them. For example: 1. Процесс протекал нормально, но внезапно (more likely) произошла ошибка. (A process went normally, but suddenly an error occurred.) 2. Когда учишь язык, ошибки (more likely) случаются. (While learning a language, errors happen).

They are synonyms. happen is derived from Middle English "hap" occur is derived from the Latin word occurrere English has a lot of such synonyms of different origins. For example: 1. oversee, derived from Middle English "oferseon" supervise, derived from the Latin "supervidere" 2. wonderful, derived from Old English marvelous, derived from Old French

Counter-question: is there any difference between "occur" and "happen"?