Microsoft has made the Windows RT 8.0 recovery image available for people who are having problems upgrading to Windows RT 8.1. You can use this to return your Windows RT computer to a functional state by booting of a USB drive that you prepare using the image. When booting, hold the volume down rocker, press and release the power button, and when the Surface logo appears, release the volume rocker. You'll then get the option to click Troubleshoot. Most likely you'll want to open a command prompt and issue the bootrect /rebuildbcd.
You a find more details, including detaied recovery instructions, at: http://www.microsoft.com/en-us/download/details.aspx?id=40810