XPS power system components

The only replaceable "power block" is the AC adapter.  Check it - make sure the LED on the DC plug is on before you plug in the notebook end.  If it's not, the adapter is bad and will need replacement.

Any more than that and it's a failed component or components on the mainboard, and you're in for a cost analysis.  Many of the components are indeed inexpensive - it's finding what's bad that's the expensive part from a labor standpoint.  You'd need a microscope and test equipment to begin with -- and even if you identify the bad parts, you'll likely need a donor board with known working components from which to salvage the replacement parts.  There are shops that can do such repairs as well, but unless the system is fairly new (under three years old) it's likely going to be less expensive to simply replace the mainboard.