Deleted Operating System on imac
I have been given a 2009 imac from a member of my family, I had to reset it as it still had all their old stuff on it and was signed into their appleID.
While resetting it I think I have accidentally deleted the operating system and now it is only showing a white screen with a flashing file & question mark symbol.
Before I reset the imac it was on macIS Hight Sierra.
Any help would be much appreciated.
iMac