As people have said before, it will just shutdown and reboot, as though the "excavate command doesn't have a "startup" function, but you could always reset the turtle by placing it at the start and run the program again. For example; excavate 20
Just place the turtle BACK to the spot you originally placed the turtle and do the "excavate" command once again (same length) and it should just zip through the gaps and quickly continue on its marry way.
You could also start the turtle where it finished off at the start, just put it adjacent to the location of the placement of the turtle and run the "program" again, and it should reach the place of stopping quicker.