Power outage affecting components?

when i came back from dinner, there had been a power outage but my computer was switched off but the PSU switch was on the ON on and as i sat down when the electricity came back( bear in mind that i was wearing shoes so i dont know how this happened), i was getting a slight jolt when i was touching the casing even while wearing my shoes( note my house is not grounded so unless i don't wear anything on my feet, i will feel a shock). What could have happened? Also what are the dangers of a power outage and what components can it affect?