Try holding the power button for a max of 20 seconds. It should only take 12 seconds generally, but hold it til it shuts off. This has happened to me as well. You just gotta shut it off and save your games more frequently. Make sure your battery isn't inflated as well.

If you have compressed air in a can, spray it along the whole 3ds hinge (If you have a 2ds let me know). A magnet in the hinge determines whether the 3ds is open or shut. Don't have compressed air? Dry your mouth as much as possible and blow forcefully along the hinge.

yes i have a 2ds