If it is a 2010 or older, then you will need to get a physical install disk as the older Macs cannot use recovery. Apple sells a Snow Leopard 10.6.3 disk in the online Apple store - that will work if yours came with an OS earlier than that.
Put OS on a Mac again.
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how do i install an OS to an older 27 inch Imac. it has no drivers, OS, browsers and can not boot into disk utility.