never happened before, wonder how much trouble I am in?

I am an LPN over age 60, retired. Just took a LTC job a day or two a week and am in last week of orientation. A few minutes ago at 5:45pm I got an indignant call from DON wanting to know why I am not there this evening. I stammered my apologies and got out the xeroxed copy of the schedule there is nothing written in for this evening. I told the DON this and I could tell she does not believe me. Unlike the full time persons I am merely written in for the days I am supposed to be there. No slashes on the days I am not working. I called a friend at the facility who sneaked a peek at the master copy. She says a very faint 3 is written in the spot but you can hardly see it. I suspect that the number is so faint it did not show up at all on my copy. I wonder how much trouble I am in? The DON says the employee is responsible for making sure their copy jives with the master copy. I guess she has a point and I don't know what they will do to me over this since I am still on oreintation. Darn, I really liked this facility.