Since you have no power either via power adapter or tv USB port, it's probably not a problem with the power adapter but that's one of the easiest things to check. Most of us have old USB power adapters around from other devices. Just be sure the fine print on it says it has at least a 1 amp output
That leaves the Roku itself or the power cord.
For the 3810 Streaming Stick+, that power cord is a special one that contains the 3810's Advanced Wireless Receiver (i.e. the wifi radio), and the USB power cord extension if you are using it.
The extension is a fairly cheap component you can find locally to replace for troubleshooting. The AWS cord costs about $20 and that's a pretty expensive purchase just to find out that is not the problem.
If replacing the power adapter and the power cord extension don't get things working then I wouldn't go any further and I'd just replace the whole thing.
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Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X