Start Block Disappeared

In one of my procedures, the start block disappeared. I had a block set to remove an item from a player's inventory, but then I deleted that item. I went in to change it to something else and that worked, but the next time I went in to the procedure, every block including the start block was gone. There was an error message at the bottom of the procedure page like if you had an empty minecraft block but not that. I tried to save it and leave, but it said there was an error, and it would not let me. I just recreated the procedure, but I would still like to know what happened.