When one gets hacked.
IMMEDIATE PRIORITIES
Contain it, so other accounts are not affected Establish what happened Inform the client Clean it properly
CONTAINMENT
If the site is sending spam or serving malware, taking it offline temporarily is appropriate.
Tell the client before or immediately after, with the reason.
INFORMING THE CLIENT
Promptly and plainly. What happened, what you are doing, what they need to do.
Do not conceal it. They will find out, and concealment is worse than the compromise.
CLEANING
Restore a backup from before the infection, then update everything immediately.
Without a clean backup, replace core files and extensions with fresh copies and remove anything unaccounted for.
THE ENTRY POINT
Close it, or it recurs within days. Usually out-of-date software or nulled themes and plugins.
WHO PAYS
Depends on your agreement. If you were paid to maintain it and did not, that is yours.
If they declined maintenance, cleanup is chargeable. State that in your terms in advance.
AFTERWARDS
Review whether other client sites share the vulnerability.