Been having this issue every now and again on my old 3ds XL. Screen will sometimes be black and console generally unresponsive when waking from sleep. Closing the lid doesn't put it back into sleep.
Forcing a poweroff by holding the power button and turning the console back on brings me back to the home menu fine, so not a massive issue, but quite annoying (especially if this happens in the middle of playing a game).
Haven't tried letting the console sit open yet, but I am trying this now. I'll let it sit open for 15-30 mins and see if the console sorts itself out.
Anyone experience this and/or know a fix?