You've screwed up big time by messing with the wrong file. C:\xboxdash.xbe is the file that triggers the softmod, and now that you've renamed and replaced it your Xbox isn't softmodded. If it was modded with a gamesave then you could run the game, load the exploited gamesave, and use ftp to undo the damage you have done. Once you get it working again, grab HeXEn (Google Heimdall's Xbox engineering Disc), burn it to DVD-R, and use it to upgrade your softmod to a recent and much better one - Krayzie 1.1.1 or Kingroach NDURE 3.1. HeXEn also gives you tools to easily change the dashboard, which I will talk you through when you get to that stage.
If the gamesave isn't on there then you need to start from scratch - a new gamesave and a whole pile of manual work, or hotswap and a whole pile of manual work.