bhoduarts

In cases like these it's a good idea to change all your passwords, including hosting/email logins as well.
 
Finding the point of entry make have been something in WP or it could have been executed via another (insecure) script on your server. In some cases it may be down to poor overall server security which is your hosts responsibility.
 
In any case contact your host as they should be able to help you identify how the hacker got in.

i usually save incremental backups: filename 1 , filename 2, ... and so on then you can back later and delete the earlier versions. sometimes when an app crashes it can corrupt the file and if you only have one version of it then you're buggered