If your camera is still covered under the Canon warranty so if these procedures are unsuccessful, you can send it in for free service using the link below.
There are a few things that can cause this error. If the flash does not "pop up" when the camera is used in low light (depending on the mode of operation) Err 05 may be displayed. First, there is a known problem with a sensor switch located in the hot shoe (external flash mount). If this gets stuck, dirty or blocked, the camera thinks that an external flash is mounted and stops the "pop up" flash from operating correctly. Use a toothpick to exercise the switch pointed out in the picture, Also, use a tooth brush to clean under the slides on the right and left sides to clear the springs. (see attached photo #1) Try this procedure first, take a few shots and see the result.
Next, the spring latch on the pop-up flash can also stick. It is part of the hinge in the rear. You need a stiff business card or maybe one of the thin plastic ones that come in credit card offers. Use it to gently lift the flash while it is in the process of clicking. You can try to gently raise the flash and while I would not suggest that you force it open, try to get the card under it while it is clicking and help it up. If you are able to get the flash to pop up, you need to clean it with alcohol and lubricate the latch and spring with WD40 using a q-tip, since dirt and grit can block the correct operation. (Photo #2)
There is also a sensor switch at one of the two points shown below that tells the camera that the flash has popped up. Once up also check these.
Please keep in mind that my diagnosis & solutions provided are directly dependent on the accuracy of your description of the problem and that resolving this issue can take multiple attempts. As with any "do it yourself" fixes, success is a "team effort", since I can't see or touch the camera, and relies on the customer's manual dexterity and ability to follow the instructions well. If the "do it yourself" approach doesn't work out for you and you need conventional repair, please let me know.
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