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I have turned my IMac on, and it as booted up to the login screen I enter my password and I just get the rainbow wheel spinning, and doing nothing else. Can anybody please advise me on how I can fix this problem please

The first thing I would try is doing SMC and PRAM/NVRAM resets. How to reset the SMC, PRAM and NVRAM on a Mac Desktop If those do not work, your Hard Drive might be getting full. How full is your Hard Drive?