Umpire Interference

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What if the umpire runs into the catcher while she attempting to field a throw with a runner coming into score? You can't assume an out, but in my scenario, the plate umpire definately prevented the catcher from catching the ball - leading to the ball getting by the catcher, and the runner being safe. Would the runner(s) be placed back on the bag that they started from? If so, would this be the bag they occupied at the time of the pitch, or the last legally touched base?