Occasionally, after installing a new WordPress plugin you can get a 500 internal server error. This problem is compounded by being unable to access any page on the site. This includes both the front facing pages as well as the back-end administration section used to install and uninstall plugins.
I recently had this issue after installing W3 Cache plugin. I did a quick google search and came across a lot forums asking for help on this exact issue. It took a lot of shifting through different forums to determine that it was not going to be as simple as just deleting the specific plugin folder. Turned out the repair was almost that easy.