If you lost admin access to a page, or if you find a page that should belong to your business/organization but is owned by another user, you can start an admin dispute with Facebook.
In order to proceed with an admin dispute, you must provide the following documents:
A notarized and signed statement from a person with sufficient knowledge and authority over this matter that includes all of the following:
a) A description of your relationship to the Page (and authority to request a change in the person(s) who manage the Page, as applicable)
b) The name of the current person(s) who manage the Page, as applicable
c) The relationship of the above person(s) to the Page;
d) An explanation of your request, and whether there has been a termination of the employment and/or business relationship with the named person(s), as applicable
e) All documentation supporting your request, including a copy of a valid government-issued photo ID (such as a current driver's license or a passport) of the individual signing the statement. See the different kinds of IDs accepted by Facebook HERE
f) The Facebook account or email address associated with the Facebook account (or Timeline) that you wish to have added as the new admin of the Page; and
g) A declaration under penalty of perjury that the information you have provided is true and accurate (your statement must include this language/exact declaration).
Once you have provided this information, the internal team at Facebook may follow up with additional requests for information or documentation to evaluate your request.
Providing the requested information does not guarantee that Facebook will be able to fulfill your request. Keep in mind that, if they are able to process your request, only one admin will be added to the Page, and it will be the admin's responsibility to take action and manage the people who work on the Page.
You need to have a statement describing the situation, who you are, and why you're the rightful owner of the page, with all the details listed above. Then you need to sign it in the presence of a notary public, who needs to co-sign and confirm the document with their seal and their registration information/stamp.
Then that document needs to be scanned into one file/pdf/document that also includes your ID, to confirm the signature. It's important for everything to be contained within one document, so that when it goes to Facebook's internal team they can see that everything is connected, legitimate, and in order.
...If the page is an unofficial Facebook page, sometimes there's an option to merge the page into an official page. This is not always available; it's currently a trial rollout.