No power to anything on ignition system when turning the key

The larger starter terminal on GM cars is used as a tie point for other electrical circuits. You'll find some small wires bolted to that terminal along with the fat battery cable. Those small wires start out with a short "fuse link wire" spliced in. Those are a smaller diameter than the wires they protect so they're the weak link in the chain, and the insulation will not melt or burn.

You test a fuse link wire by pulling on it. If it's good, it will act like a wire. If it's burned open, it will act like a rubber band. Auto parts stores have replacement fuse links. You'll get a piece long enough for two or three repairs. The color is chosen according to the current rating. Common colors are dull green, gray, black, white, and orange.

How did you determine that? Did you disconnect a wire from someplace or was the rest of the circuit still connected? Did you use a digital voltmeter or a test light?

Keep using the test light because it draws current to work. I've seen very experienced co-workers make a wrong diagnosis when using a high-class voltmeter, because those draw almost no current to do their thing. A burned-open fuse link wire will leave a carbon track behind where it arced when it burned, and that carbon will conduct enough current to allow the voltmeter to detect a voltage, ... As long as that circuit isn't loaded down with its normal stuff, like a heater fan motor. That falsely tells you the wire is good up to that point. You won't make that mistake with a test light. You won't be able to get enough current through that carbon to run the light.