Won't unload from the trailer!!

[barrelracer815]

I have a 4 year old mare that is really riding well and going great on barrels.  She hasn't shown any sign of soreness problems when I ride her or do ground work with her.  However, when I went to unload her from the trailer one day she wouldn't go.  I couldn't figure out why she would do this all of a sudden, because she has always unloaded before.  Do you have any ideas as to why this might be??  What can I do to fix this?