AbleCommerce Shopping Cart Software

I started getting this error today. OnePageCheckout hasn't been altered since the site launched in June, so I don' t know why I'm getting this error. Please help!

Details:
[[ConLib:OnePageCheckout EnableGiftWrap="False" EnableShipMessage="True" MaxCheckoutAttemptsPerMinute="20"]] c:\websites\AC702\ConLib\OnePageCheckout.ascx.cs(1168): error CS0012: The type 'CheckingOutEventHandler' is defined in an assembly that is not referenced. You must add a reference to assembly 'App_Code.xhrftebv, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.

I am running AbleCommerce 7.0.2 build 11659

I found this post: viewtopic.php?f=44&t=9620 , and while replacing OnePageCheckout.* didn't help, replacing App_Code/CheckoutEvent.cs resolved the issue. I still don't know why this happened, however.