You will have to open the radio and start checking for any obvious problems, preferably with a DMM (digital multimeter).
With the power disconnected (also remove the backup batteries, if any installed) and after the radio has been opened, use the DMM’s Ohmmeter function to make sure that the power cord is OK from the plug to where the cord terminates in the radio.
If the cord is OK check if there is a fuse mounted on the board close to where the power cord is terminated and if so measure it to make sure that it is OK i.e. it should be 0.00Ω - a short circuit.
If the fuse is OK check that there is continuity from the fuse to the Power button and that it works OK.
After that you will have to do point to point testing through the board to find out why there is no power getting through to operate it.
Without a schematic of the board it is difficult but doable. However you need to know how to use a DMM and also what the various components look like and what they do in the circuit e.g. resistors, diodes, capacitors, ICs etc.
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