The Power button is part of the keyboard assembly.
Here’s the maintenance and service guide for the laptop.
Go to p.86 to view the necessary pre-requisite steps and then the procedure to remove the keyboard assembly. You will probably have to do this to access the power button.
If you can’t fix it then you may have to replace the keyboard assembly. Sometimes the Lenovo part number can be found somewhere of the underside of the keyboard e.g. 04X1277.
Search online using the part number only to find suppliers that suit you best.
There are 194 keyboards available for the model. They vary as to the country and language where the particular laptop was made for. If there is no part number to be found on your particular keyboard, here’s a parts list that may help. Click on Select Commodity > Keyboards Internal (194) to find all the part numbers for the various keyboards available. In the keyboard part name the country code is shown e.g. USI = United States International, UKE = United Kingdom English, SPA = Spanish etc