Have you checked with your SharePoint site administrator?. They might have made some changes or put restrictions in place for a specific purpose. So, shooting them an email or giving them a call should be your initial course of action.
If you have the appropriate permissions, take a peek at the site settings and permissions. Double-check if your user account or the group you belong to has been granted read-only access. Sometimes, permissions can get a bit wonky, and a simple adjustment might do the trick.