⬢Check the switch position on the side of the PSU if it's in correct position.
Make sure that the power supply is not connected to mains!
Correct position means that the switch is on the side with your mains voltage. (If you have 110/120 V mains [mostly America] the switch has to be on the left, if you have 220/230 V [Europe and the rest of the world] the switch should be on the right ).
You can change the switch position with flathead screwdriver.
This is very important part, if the power supply is configured incorrectly it can be damaged!