If you are using UMDL (not the official updater tool), you will plug in the phone while it's turned off and then turn it on, which will start the flash, so you are bypassing the current ROM on the phone and WMDC.
Grab the IMG file from
here. You should already have UMDL if you have been flashing ROMs but it can be found
here along with the USB drivers. Follow those steps and you will be back to stock.