The Surface forum would have been fine too, but this works.
You can try booting to the built-in recovery mode. However, it's possible that the recovery partition was removed to free up space. No worries, though! All you need is a bootable flashdrive with a recovery image. The files needed are available on this site (check the Surface sub-forum). After you've created the recovery flashdrive, turn the tablet fully off, insert the drive, hold the Volume Down button (well, rocker), and turn the tablet on again. It should boot directly to recovery. The recovery process should reformat the internal storage, which will remove the BitLocker'd volume.