Welcome to Android Central! Did this happen at the same time? Was the same app recently installed on both?
On the one whose screen won't turn on--does it otherwise make sounds, or does it seem like it isn't powering on at all? If the latter, then press and hold the Power button for 15-20 seconds and see if that helps. I would also keep it plugged into a wall outlet and let it charge undisturbed for at least 4-6 hrs, then try powering on, in case the battery happened to drain completely.
For the other one stuck on the bootscreen, that could be due to corrupt firmware or a defective motherboard. This can be difficult to fix. One thing to try is to remove the SD card (if present), then try rebooting again.
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