Major viagra hack

Yesterday a visitor to my blog discovered that Viagra ads had hacked into my blog site — I saw it too, but couldn’t immediately act on it. A few hours later when I went back to my blog, the offensive take-over was over. But just now, when I tried to go to my admin site to create a new post, it turned out that that page was still hacked: what opens is a Canadian pharmacy for Viagra. I have no idea what to do, how to get rid of that hack & how to get back to my admin page.

Help!!!