I just got home from work, and my tort was on his back... not sure how long he was like that, how do i know he's okay?! His eyes were open when i found him...
Laying on their backs in a warm or cool spot they can survive a while without real harm, but if it is hot in that spot, like direct sunlight or a lamp, they will die very fast! Make sure you notice where he flipped over and try to reconstruct how it happened and modify the set-up you have to avoid it.