If your country uses 240V mains then flicking the switch to 115V bypassed a voltage divider and probably blew the fuse in the power supply. If you are comfortable working with mains voltage, you can pop the PSU open and check the fuse. If it's blown then just get another one with the EXACT same ratings and you should be up and running again.
If it's not the fuse then you have bigger problems and it's time to replace the PSU. Any PSU should work as long as the wattage is the same as the original or higher.
Also, you could call Lenovo to see if the PC is under warranty. If so, the PSU is probably a user replaceable part, meaning they will send you a new one to put in and you just send the old one back.
Answers & Comments
If your country uses 240V mains then flicking the switch to 115V bypassed a voltage divider and probably blew the fuse in the power supply. If you are comfortable working with mains voltage, you can pop the PSU open and check the fuse. If it's blown then just get another one with the EXACT same ratings and you should be up and running again.
If it's not the fuse then you have bigger problems and it's time to replace the PSU. Any PSU should work as long as the wattage is the same as the original or higher.
Also, you could call Lenovo to see if the PC is under warranty. If so, the PSU is probably a user replaceable part, meaning they will send you a new one to put in and you just send the old one back.