This is a very common issue with WordPress. WordPress is not that secure, and hence you must take some measures to protect your blog:
1) Always upgrade to the latest version. This reduces a lot of holes..
2) Upgrade all the plugins to their latest versions.
3) Use a strong password.
4) Do not use unknown plugins. Deactivate and remove if you are using.
Some external links to sites that give assistance on wordpress hacked sites.
Please click on the link below to read on how to secure your wordpress site againt furture compromise from hackers:
Preventing Hackers by Hardening Wordpress:
Must Have Plugins:
These plugins will enable detect exploits in your files and database(s).
Wordpress vulnerability lists:
Troubleshooting/Restoring Hacked Sites: