Hey Scotty!

My iMac has been super slow the past couple months, especially after I put Big Surr on it. Today it was frozen so I tried to restart it, but it was stuck on this screen screen (below). 

So then I tried to started by holding in shift R, and then it ultimately came up with the screen with a 🚫 instead of the 🍎. I then restart it and now it’s back stuck on the on the screen pictured below again. 

I’m assuming I need to reinstall Big Sur? Any advice?
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90% of the time, these are indicators that a SATA hard drive is failing. I don’t know what type of drive is in your Mac but boy those SATA drives are terrible. 3 types of drives:

SATA (spinning)
Solid State (no moving parts, very reliable and very fast, never see them fail)
Fusion drives (hybrid mixture of the two above. Can still fail but not as often)

On to your issue, if it’s not physical then here are a few things to try next:

Boot into recovery mode and run disk utility repair
Boot holding down shift to prevent any possible 3rd party extensions from running
Boot into Single user or Verbose mode to watch the start up process through the command line to get a possible idea of where the startup is hanging.

My guess though is that it’s the hard drive. If I’m right, then reinstalling macOS 11 will only further stress an already dying drive.