Access denied, used to be public to organisation

Hi, I have recently taken over a SharePoint page for my organisation, this was set up originally by head office and hasn't been used since. Before I was "owner" on the page I was able to view the SharePoint site with no issues. However, I was playing with permissions yesterday and I've broken it! Now, when others try to access they get an access denied message, if I turn on access requests then people can request access but I don't want to have to approve everyone, I want the site to be visible for anyone within my organisation. I've spent about 5hrs googling trying to fix this but I have no idea what to do. PLEASE HELP! Note: I have checked the SharePoint version, it is 'SharePoint in Microsoft 365'