My iMac gets to 50% complete on start-up and stalls.
I have entered disk utility mode mode via command R and ran first aid on the hard drive. I have erased the hard drive and reloaded MacOS with no luck.
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Try resetting the NVRAM per How to reset NVRAM on your Mac and if that does not work try restarting in Safe Mode per Use safe mode to isolate issues with your Mac - Apple Support.
If neither work, your internal drive has likely crashed and needs to be replaced. Take it to your local Apple Authorized Service Provider or Apple Store.