ByteBoi 2.0 keeps its games on a micro SD card, so both of these messages usually come back to the card rather than the console.
If it says "No SD Card"
Switch the console off, take the card out and put it back in until it seats fully. A card that looks inserted but has not clicked home reads as missing.
Check the card is the right way round against the build-guide photos at circuitmess.com/blogs/resources.
Try the card in a computer with a card reader if you have one. If the computer cannot see it either, the card itself has failed — tell us that in your email, because it changes the answer.
If the games are gone, or you have lost or replaced the card
The games can be put back. Email us at [email protected] with your order number and what happened — card lost, card wiped, or card unreadable — and we will help you restore the preloaded games onto your card or a replacement. Please do not load files from third-party sites onto the card; we will point you at the right ones.
Note that this is different from Jay-D, which has its own card rules — if you were looking for that, see My Jay-D 2.0 says "Empty!" or "Not inserted!".
If the card is fine and the console still misbehaves, work through My ByteBoi 2.0 or Wheelson 2.0 does not work after I built it and then email us with photos and a short video.