Before sending it in... remove the SD card and take a peek inside the slot.
Years ago, I once inserted an SD card backward (label should face the back as you insert the card and I accidentally put it in with label facing front.)
The card actually seemed to go in... although it wouldn't work (of course not... the gold contacts were on the wrong side). But it turns out there's a thin plastic "comb" on the edge of the SD card where the gold contacts are and these are meant to protect the card from grounding out should it come into contact with something. Say for example you put the card in a pocket that also had some coins... the coins could conduct across multiple contacts. But with the raised "comb", a metal object capable of touching one contact wouldn't be able to touch another at the same time.
It turns out, putting the card in backward caused several of the "combs" to become damaged and it left some debris inside the slot which then prevented other cards from inserting and making proper contact until I cleaned it out.
All I had to do was clean out the bits of plastic and everything was fine again (the camera wasn't damaged... only the very thin plastic "comb" design on the card was damaged.
If you do see debris inside the slot, use something non-conductive to clean it out. A toothpick, for example, would probably work well. Be gentle.
If you DO NOT see anything obviouslsy blocking the slot... it's probably going to need a trip to the Canon service center. There's no camera menu configuration option, etc. that would explain this issue so it's not like we could tell you to just reset some settings to clear it up.
Good luck!
Tim Campbell
5D III, 5D IV, 60Da
