Site got hacked

One of my sites got hacked…
If you visit the site it all look normal, also login to the control panel look normal.
But when I check how many pages of this site google crawled I got a huge number, a lot more than the real amount of posts and pages that I have in the site.
So I browsed the links google crawled an noticed many “posts” that were not made by me.
I clicked on these strange “posts” and saw that someone inject content inside a few of my posts.
What type of attack is that? any idea how to fix it?
Thanks
Sami