[Rolf]

Had a few more thoughts on what you can try... Is the door shut properly on the media card slot? That has an interlock to prevent firing... although a total disconnect usually means the whole camera is dead. Have you inadvertently got the camera in mirror lock up mode... press shutter once for mirror lock and again to fire?

You could try flooding the shutter button area with isopropyl alcohol- rubbing alcohol... carefully...that may help to wash away any residues from skin oils or other contaminants that may be on the shutter button contacts... if it does... it's a temporary fix but does show that all you need is a clean... Canon though will probably go for a replacement mechanism... that way you wont get the issue recurring!

Could also try cleaning the battery contacts in case it's a low voltage issue. Worth checking the output voltage from the battery too... swap for a known good battery and try shutter again... Clean the lens to body contacts... the camera may not be communicating with the lens too well... again with the alcohol.

Go to your Menu and try a reset to factory ... then see if the problem persists... if it does... chances are it's not an erroneous setting but points to the shutter mechanism itself... either needing cleaning or replacing... also try a different lens.... if that work OK... the original lens may have an issue...

Go to a Dealer and try a battery grip and see if the camera fires normally using the grips' shutter release...if it does and you can live with it, it may be a cheaper alternative to a strip down and clean or replacement shutter.

Can't think of any thing else... Quite a lot to consider but the whole camera is an assembly of interconnected parts each one having some affect on the other... much like the computer it is I suppose...

Kind regards

Rolf