First please check which nameserver is set in the domain. Then check what IP address that nameserver delivers when asked, e.g.
nslookup <domain> <nameserver name>
Is this the correct target IP?
Then check whether Plesk is configured to react on that IP (Tools & Settings > IP Addresses).
Then check what IP the domain is configured to use.
Is this all lined up? Correct nameserver? Nameserver responding? Responding with the correct IP address? Plesk knowing that IP address? Domain configured on that IP address?