The PSU is dead.
Replace it with one that has APFC (i.e. Active Power Factor Correction). PSUs with APFC don't have a little red input voltage switch, they use automatic input voltage detection to avoid the exact user error that you've experienced. Any PSU with any level of 80 PLUS certification (i.e. 80 PLUS, 80 PLUS BRONZE, 80 PLUS SILVER, 80 PLUS GOLD, 80 PLUS PLATINUM, 80 PLUS TITANIUM) will have APFC.