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How old is the camera in question, please?
And, how old is the battery you are using in it?
Does the lens assembly move when you switch on the camera?
Given that the battery is that old, you probably have a battery/batteries that are getting weak.
Rechargeable batteries do get weaker with time, and eventually fail to deliver enough power to start up a device like a digital camera; they then need replacing.
The replacement battery needs to be charged up for 6+ hours before its first use, owing to its having been on a shelf for a considerable time.
Try that, I advise.
Also try cleaning the terminals in the battery compartment, after removing the batteries, first by rubbing off with a fresh pencil eraser, then by wiping the terminals off with cotton swabs moistened in rubbing alcohol, then letting them dry off for 5+ minutes with the battery compartment door open, then try turning the camera On again.
Whiskey, if not too expensive, will do as a grease/oil-remover in a pinch, for lack of rubbing alcohol.
Enough to dampen a swab is sufficient and probably costs no more than a quarter or so.
Give it several minutes after using solvent (or, Whiskey) on terminals, for evaporation time.
ISO refers to your camera settings.
The ISO setting.
"Busy" - that's not so good. Wait a while. See if 'Busy' goes away, then see if you can shoot pic.s.
Try 10 minutes. After that, waiting longer won't do anything.
OK. Let me know how it goes, please.
Then the camera, unfortunately, may well be 'broken'. It is, after all, about 4 years old.
To have it service/repaired, either send it to Canon, or perhaps to PrecisionCamera or to UnitedCamera, which also service Canon high-end DSLR cameras such as yours.
To have it serviced by Canon... are you located in Great Britain, please?
Well... does a picture get taken? it sounds as if it does not.
Then you have a defective memory card, quite possibly.
If it continues working, with different memory cards, I hope that we can consider it 'fixed'.
Well. Good. It seems to be quite thoroughly functional.
The 'bad' memory card might be tested briefly in a computer's card reader, and images perhaps recovered from it, but is most likely defective and damaged. Note whether its contacts (metallic little bits) are in any way dirtied, corroded, bent, missing, etc.
If this case is completed, I'd appreciated a rating for my work on it. Thanks!
Good! Glad to hear the problem is cleared up in good style.