Netflix codifies all errors you may encounter during viewing, with distinct alphanumeric codes for each error regardless of viewing platform. Error code N8109 is an error unique to browser-based viewing and relates to Microsoft Silverlight, the application framework that supports the Netflix application online.
Error Code N8109 refers to an error with Silverlight, typically stemming from Silverlight being out-of-date. Since Netflix relies very heavily on Silverlight for browser-based viewing, being out-of-date or having a corrupted plugin can very adversely affect playback or inhibit it entirely.
Silverlight is an application framework for rich Internet applications. Similar to Adobe's Flash environment, Silverlight is primarily used for streaming media and websites displaying multimedia, high-end graphics and complex animation. All three of the major PC Web browsers support Silverlight, namely Microsoft's own Internet Explorer, Google Chrome and Mozilla Firefox. Mac users running Safari or Firefox can also enjoy Silverlight content.
Like many Microsoft products, the Silverlight plugin used to run Netflix can be repaired using a Fix It Tool. Fix It Tools are specialized programs developed by Microsoft to diagnose and repair any errors, corruptions or other issues that may be inhibiting the function of a program. Download and run the Fix It Tool (see Resources) to try and fix the issue -- if it doesn't work, the issue may require a clean installation of Silverlight.
Your Silverlight plug-in may be so outdated or corrupted that it cannot be fixed. In these instances, removal and installation of a current version may be the only option to restore Silverlight functionality and get Netflix working again. Close all open browsers, hover the mouse over the bottom-right corner of the screen until the Charms menu appears, and click "Settings | Control Panel | Programs | Uninstall a program." Look for and select "Microsoft Silverlight" from the program list and click "Uninstall." Once the uninstall procedure has completed, open a browser and navigate to the Netflix Watch Instantly page (see Resources) and click "Play" on any title. You will be prompted to download and install the latest version of Silverlight onto your computer, which should resolve any further issues.