You may find your domain gets compromised at some point, this guide goes over how to proceed in that situation.
The following site might help you track down the source of the hack:
We advise that you begin the process of removing / cleaning these files as soon as possible , patching and securing the PHP code and applications or plugins, and resetting FTP / admin passwords.
If you are not familiar with cleaning a compromised domain then you may need to get someone to look into it on your behalf. Another options would be to see if we have a backup available for the domain, as our backups are for disaster recovery purposes only there is a fee of €50 associated with their use.
If you decide to go with the backup you should bear in mind is that all it does is restore the domain to an earlier version, it won't remove whatever weakness allowed it to be compromised so you'd still need to ensure the application is up to date, along with any plugins/themes/modules it may be using. If you are still interested, then e-mail email@example.com asking for a backup of the website files and databases and specifying a date you would like it to be restored from.
Most compromises occur due to out of date applications or out of date plugins/modules/themes the application uses. For that reason it is imperative that you keep your websites up to date to avoid a disruption of service. Prevention is better then a cure.