Hello. My name is***** can assist you with your question.
If the lens is not going back it all the way, then he should be giving you an error and not really powering up completely. If this is the case, then the lens itself is damaged and it needs to be serviced. The lens is a very delicate mechanism and something as small as a grain of sand can cause it to stop operating. If it has been dropped and something inside the lens assembly has been that, it will need to be replaced for the camera to operate at all. Simply putting the ring back on will not make it work again. Please a me know if you have any questions.
Ok. Thank you for the additional information. You CAN replace the lens assembly. This is really the only way to correct the problem - they don't sell the lens cover or ring separately. It is not expensive, though. The genuine part is around $20-25. You can get a third party manufactured part for about $15. Here is a link to the Canon part:
Let me know if you have any questions.
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