If you're using Wordpress, someone(more likely a bot/script) probably found a vulnerability in your Wordpress application. That type of stuff usually happens from outdated version of Wordpress, or even the plugins/themes.
Google does a malware check when they're indexing sites, and if they find malware then they put this message across the site.
The best thing to do now is see if you have a backup of your site that you can restore(your host might be able to get one if you don't have one". If you don't, then you will have to manually check through all of your files and remove any malicious code. It's not too hard because you can look for the "Last Modified" dates to see files that have been edited recently that wasn't from your own doing.
Once you have done the above, it is then recommended to updating all passwords associated with your website like FTP, Database, WP Login, etc. Then proceed to update Wordpress and any plugins to the newest versions. I also tend to do other things to secure my Wordpress such as doing an .htaccess password protection for the entire wp-admin folder. That way they won't be able to go through the directory at all and they would have to break through two directories.
It might also be a good idea too... if you do have a backup, that you delete all your files first, and then restore the backup. Just incase you have some type of backdoor script hidden in there. You could be restoring a backup and it will only copy over all of the files that were originally a part of your website. Just don't make the mistake of deleting it all if you don't really have a backup. There might be some tools online like can scan your website and maybe even for backdoor scripts if you just restored a backup, but I'm not allowed to post links anyways.
Edit: A lot of people tend to blame the Web Hosting company. But it's rarely ever the case. I used to work for one of the biggest Web Hosts in the world and would get calls from people who had this same issue, and they would try to blame us. I would explain... "we're not getting any other calls from our million+ other Web Hosting customers... why would someone hack into our servers, so they can target your website only?". Then after restoring their site, it was way outdated, and sometimes could even replicate the hack to do it again. The fact is, this happens alot with Wordpress, and if someone is going to use Wordpress then they will need to regularly check and update their website regularly. I don't know about lately, but they have always constantly updated it to patch up vulnerabilities. Just thought I would mention that because it sounds like people might try to tell you that.