My WordPress Site has been hijacked and I need help!?

I downloaded a popular plugin that has apparently also been hijacked, Maintenance Mode. My site started redirecting to brend-store.ru and then to malicious scareware on www1.squadroshield.co.cc. Now, none of my pages work and all show 404 errors, even though they still exist. This is extremely detrimental to my business and I need help!