Follow this procedure to resolve issues related to power.
System error LED is on and event log "Power supply has lost input" is displayed
The power supply is properly connected to a power cord.
The power cord is connected to a properly grounded electrical outlet for the server.
Make sure that the power supply AC source is stable within the supported range.
Swap the power supply to see if the issue follows the power supply, if it follows the power supply, then replace the failing one.
Review the event log and see how the problem it is to follow the event log actions to resolved the problems.