There is very little difference between the two and either could be used in any context.
"Someone stole my car." is likely to mean yesterday or the day before or somewhere more definitely in the past.
"Someone has stolen my car!" would be more likely to be used when a person first notices that his car has been stolen.
The English verb system with its multiple verb tenses probably derives from the French. An interesting comparison is the Russian verb system in which the main idea is aspect (action at a definite time or continuing action) and not tense.
You can actually mix verb tenses in Russian without causing any alarm:
"Иван вышел из машины и идет в магазин" (John got out of the car and walks into the store) is horrifying in English but perfectly good in Russian, the first verb sets the time and the rest of the sentence is just telling a story.