Or log in with a secure shell session (ssh) and navigate to the shell/ folder, run compiler.php with the following command
This disables the compiler and empties the compiler code cache.
php -f compiler.php clear
Then go delete all the subfolders in var/cache.
Disabling the compiler, emptying its code store and manually clearing the Magento cache should bring your system back up. Once there, check to make sure the new extension is functioning with your website for a few hours and then see if you can reenable the compiler.
When installing ANY module, check that it actually is listed as working with your Magento version before installing and then observe the following.
1) Log into the admin backend, Flush and disable the Magento Cache.
2) Disable the compiler and flush its code store.
3) Log out of admin and delete all files in var/session.
4) Install the module.
5) Log into the administrative backend and when the backend finally shows, log back out.
6) Log into the administrative backend and go check module configuration settings for your new module.
7) Once you are sure things are working, log in and reenable the Magento Cache.
8) After running the site and making sure the new module is functional and living well with your code base, try reenabling the Compiler and recompiling. Be prepared to manually disable the compiler if things go south because a module isn’t compatible with Magento’s compilation process. In shell/, manually execute php -f compiler.php clear to shut it off, then consult with the module’s creator to see if there’s an upgrade available.
Module installation and first run is an operation that must not be interrupted by cached code, sessions referencing old code.