Hello, Canon user of almost 15 years here. I used to leave whatever lens I lhad ast used on the camera while I wasn't using it (the 5D mkii & mkiv). Recently I got a pellican case and for storage purposes traveled with the lenses off (and body caps on). ...When I reattached the lenses (properly, nothing weird) the camera would not fire. Everything else as it should be. Shutter would not go. There may have been an error message but I honestly can't remember. This intermittently happens now once when I was about to use a telephoto for a kiss at a wedding (I'm a wedding photographer). STRESSFUL! And defeated the purpose of traveling across country with a MKIV (I am currently using the EOS r as well) and 70 - 200.
Anyone else experience this? Is this a firmware issue? I have never had this happen before this year.
This may be a case where CAREFUL cleaning of the contacts on the camera body and lens would be helpful, it is possible that something got onto the contacts while the lenses were being stored or during transport. If the camera cannot communicate with the lens, it will assume there is no lens attached.
Canon recommends cleaning only with a soft cloth. There are other methods for cleaning electrical contacts that have been contaminated but if you are not already familiar with this process, your camera and lenses are not the place to learn. Get them to Canon for this sort of service.
In the area of probability it is not a firmware issue and it is not a cleaning the contact issue. Other than the AF/MF suggestion I really don't have a specific cause but the likelihood of the first two is very small.
Do the normal reset to factory default settings. Menus, tools, clear all settings and clear all custom settings. That's about all you can do on your own.