Short..

We've had a similar case like this a few weeks ago, where a customer has multiple PSUs on the desk and just happened to pick the wrong one: A 19V Laptop PSU. If the same has happened to you , the good news is that the protection diode did it's thing and protected other components from more damage.

The protection diode in question is D8, type is 1SMB5927B (complete number we've used is 1SMB5927BT3G, but that extension is only for "tape and reel" packaging, which you don't need for a spare part). This is a Zener diode that will short anything below 0V (any negative voltage), and it will also short anything above 12V. If it gets overloaded, it usually completely shorts out, and all current power supplies just stop sending power in such a case.

The diode is located near the swich. The computer will work without it, but you have to keep in mind that it's completely unprotected. You should double-triple-check that the power supply you're trying has the correct voltage and polarity. If it works, do place your order for a new diode and solder it in, so you have continued protection of your precious chips.

The whole idea of this diode is to limit damage in case of a user error. It's not guaranteed that it worked, but so far, it has worked in all cases where we had a board back for repair. Luckily, these were only a few since introduction of the C64 Relaoded MK1 in 2015 (yes, that had the same protection circuit).

Please report back with your (intermediate) results.