An acute stress reaction is a psych problem that happens after a trauma (abuse, death of a family member, car wreck, etc) Symptoms according to most diagnostic guidelines include immediate reaction to the trauma (feeling of numbness, derealization, etc) basically an initial state of daze. I'm not a doctor, but I'm pretty sure most psychologists (like my mom) would probably consider the victim having PTSD (not "acute stress disorder") if the problem persists after a long time. Usually a disorder shows a pattern of the symptoms happening more than once.
=) I did my best trying to explain, but if you need more info you could look at a local library and pick up a couple of psych books. Hope I helped =)