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I would try using the Reset Hole on the left side of the device (between the microHDMI and microSD card). Press that for 3-5 seconds. It is possible you got into APX mode (for transferring files with the device not in the OS using NVFlash. Great if you bricked a device). APX mode is Hold VOL UP and tap POWER. Same way to get out, but does not always work.
Also, try holding the power button for 30 seconds, release, hold it for 3 seconds and see if it boots.
Finally, if that does not work, hold VOL DOWN and POWER for about 12-15 seconds until some small text appears in the top left.
Here is a list of more tips and tricks, but I think I pulled all the relevant ones out in this post.