OS X Big Sur lost afyter battery replacement. HELP!
Following your excellent video to the letter, and being used to working with electronic domestic appliance repairs, I found after calibration the Mac shows the grey '?' file. Apple Support advice did not fix the problem. It seems I cannot reinstall from Time Machine as there is NO OS X Big Sur to reinstall. Various fixes just could not find it, or the link I needed was greyed out. Is my Mac now terminal, or can I install from a disk? Your help and advice woud be much apreciated.
Thank you.
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