Have Skyrim, purchased through Steam. Was running fine. Using MMO, I was made to create an Overwrite Mod. While cleaning out some old Mods I no longer use I wasn't thinking and deleted the Overwrite Mod. This, in turn, deleted my Update.esp. I'm not enough of a techie to understand why or how that happened. Anyhow, following the advice here I poked around a bit and did what a few recommended - verified integrity of game cache. It informed me that files were missing and downloaded them, fixing my problem. To do this, right click on Skyrim in your games list in your Steam homepage. Click on Properties. Click on Verify Integrity of Game Cache. Let it do it's thing.