SMPS stand for Switched Mode Power Supply. Its basic function is to rectify the external Ac(Alternating Current) supply to DC(Direct Current) so that it can be distributed among the different parts like motherboard, harddisk, Disk Drives etc as almost all these peripherals are generally 9V, 12v, 15v or just nearby. It clearly means that if your SMPS is not working then you cant even switch on your computer.
Now talking about the problem with SMPS, it can be because of voltage fluctuations in the power supply, short-circuit, some physical defect or any other reason.
I will suggest you to first check its manufacturer's warranty. If your SMPS is in warranty period then you can get it replaced easily. If not, then rather than repairing it I will suggest you to buy a new one with higher voltage capacity and long warranty period.
Hope for the best.
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