I would suggest connecting to your website's server via an FTP client or through your hosting company's file manager. Then, you should be able to access your plugins folder and delete the W3 plugin to regain access to your site. The plugin folder location should be something like "root/wp-content/plugins".
There are a number of things that could cause this problem. Depending on what theme and other plugins you are using, the W3 plugin might be incompatible with something. Once you have deleted the plugin from the folder, you should regain access to your website. Once back into the dashboard, I would suggest deactivating all other plugins, then reinstall the W3 plugin and make any desired changes. Once you have saved any changes, try reactivating any other plugins one at a time to see which one, if any, might be conflicting with the W3 plugin.