turn the key in the on position and nothing happens, no lights come on. nothing at all will work.
With the limited info you gave us I would start with the battery, battery cable ends (terminals), battery cables, fuses, main fuse, corrosion, voltage drop test, loose connections, etc etc…
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Call your local repair shop, ask for their advice. They’ll probably just want the car towed to their shop.
If you are asking what a typical driveway diy’er could do in this situation, me, I’d start by measuring the battery voltage, and verifying the cables that connect to the battery posts are secure. If you post what you discover here, you’ll likely get some more ideas.
Did you try anything ? Check the battery , cable connections for corrosion . Trailblazers were also a bit notorious of getting a bad ignition switch specially if you are someone who likes to carry a dozen keys on your chain .
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Waiting to see if the OP comes back before investing my time beyond this-
Get in the truck and try turning on one of the courtesy lamps over the rearview mirror. Does it come on?
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as stated above- check the battery. If this is side post battery that Chevy loved to use, they are bad about corrosion and also about coming loose all the time.
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It’s not easy to check on these trucks. The battery is completely covered in a plastic shroud that is held underneath a corner brace. I’ve had three of these Trailblazers and after the first one, I toss the shroud if I ever have to remove it to gain access to the battery. PITA.
I hear a lot of complaints about the side terminal connectors but have never had any issues with them on any of the many GM vehicles I have owned. I must lead a charmed life 
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A battery can go completely dead in the blink of an eye.
Tester
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And with 0 warning…
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+1
It happened to me with my '86 Taurus. I drove home for a very brief break before an evening meeting, and when I tried to restart the engine, the 4 year old battery was dead. Luckily, in those days, AAA responded very rapidly.
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A few years ago my truck had been cranking more slowly than normal. When I investigated, one of the circular connectors that attaches to the battery post had corroded enough to eat completely through the tightening bolt. If I hadn’t noticed and replaced the failed part, my truck would have developed the same symptom as OP is describing.