[sassan]

Andy gave a percise and good explaination.
If seen again, you want to turn off the camera and then on if still see it, open the battery compartmet and close it. Usually you are back to business at this point.
IF you use an eos lens EF EFs that generates the problem, you need to clean the contacts with a pencil eraser. Be gentle.
If you have a Sigma that cause that problem, its the old inner chips (Incompatibility that needs a new chips installation by manufacturer) and if you use a TC attached to and old FD lens via adapter, you need to tap the contacts.
If non is working, memory needs a check.
Well seems your problem is solved for now but you are ready if it happens again.