When you are researching a website, you may have found that it is no longer available or that it has been altered, however this does not mean that you can't access a historic / previous copy of that website, as Archive sites, such as Archive.Org, which is often referred to as the Wayback Machine, and Archive.Today, are really useful free online resources, where you can often access a historic / previous copy of a website. Whilst they do not have a copy of every website, and are usually around 6 months out of date, they are a vital part of any online research being undertaken into a website or Domain Name.
As an example, if you were researching the owner of this website, you may come across a very old email address linked to him, of neil@crucialinquiries.com but very few other details linked to it. Obviously neil@crucialinquiries.com should lead you to the Domain Name of crucialinquiries.com, and although there is no current website displayed for that Domain, you can still undertake enquiries into it.
Obviously you could undertake WhoIs / WhoWas type enquiries, using some of the sites under the Reverse (Pre-GDPR) Whois Details Linked To Domain Names, Registrants And An E-Mail Addresses heading on the Domain Names & IP Addresses page of this website, but importantly, you can also search the Domain Name on an archive site to see what was previously shown on the website. Put the Domain Name as a URL in the search box, including the http:// or https:// and it will return details of any archived copies of the website it has, showing how many copies it has, between what dates, going back as far as 1996.
The Blue Circle on a Date show that archive has a copy of the website from that Date, and if you rest your cursor on the Blue Circle, it will display the Time it was accessed / copied. Then if you click on the Time displayed from within the Blue Circle it will open up a copy of the website, as it appeared on that Date.
The website shown will usually open displaying the main / front page, however the links to other pages within it should be active, and can be clicked on, to view them on archive. So always checkout any About Us / Key Staff and Contact Us type pages, as there is often useful information shown on them, which may then help you progress your research.