For the past week there has been a rolling brute force attack on all major host providers that specifically targets the login of WordPress sites. According to the report by HostGator, it was a well-coordinated attack with over 90,000 IP addresses involved. The symptoms are a sluggish admin area after login, slow speeds or trouble logging in.
Protect your login page with plugins such as Login Lockdown or Limit Login Attempts. And always backup your website manually or automatically via external storage (payment required).