If you are unable to edit or manage your site then almost certainly you are not signed in with the correct account - ie. an account that has editor or owner access. If you believe you are the site owner and want to identify the 'owner account' of a site, then you need to send a 'Site sharing request' for that Site which will be delivered to the owner account(s) for that site.
(To be absolutely sure who you are signed in as, see Find out who you are signed in as)
Site sharing requests are sent automatically to the site owner. This can be useful if you have forgotten the email address that was used to create the site. To send a Site sharing request, follow the instructions below.
If after sending a Site sharing request you do not receive the sharing request email, first check that it hasn't been filtered out by your spam filter. If you still have not received it, then the most likely explanation is that it has been sent to an email address you have forgotten about - check any and all old email addresses you may have.