A site you did not set up.
WHAT TO ESTABLISH FIRST
Where the domain is registered Which nameservers it uses Who has access to each
Those three determine what you can actually change.
THE COMMON DISCOVERY
Nameservers pointing at a provider nobody remembers, with no access.
That is the problem to solve before anything else.
HOW TO FIND OUT
WHOIS shows the registrar and the nameservers.
The nameserver names usually identify the provider.
IF YOU CANNOT ACCESS THE DNS PROVIDER
You can still change nameservers at the registrar, if you have that access, and recreate the zone elsewhere.
Record every current record first, by looking them up, since you will lose them when nameservers change.
WHAT TO LOOK UP AND RECORD
A records for the domain, www and any subdomain you know of MX records TXT records Anything else a lookup reveals
IF YOU CANNOT ACCESS THE REGISTRAR EITHER
That is the more serious problem. The registrant must authorise a change.
ONCE YOU HAVE CONTROL
Export the zone, document where everything lives, and secure both accounts.
