I doubt that the vents or ducts have become unhooked. It could possibly be a fuse if there is a separate rear blower and fuse on the 2001 model, or maybe a door like panel that diverts airflow in the system is stuck. I don't own that year so I am not sure. In my opinion something like this is not the easiest and takes a moderate amount of tools and knowledge to figure out. If you're still interested in fixing it I'd recommend getting a service shop manual for the van. It's about $12 for an online copy. https://www.siennamanuals.com/2001_toyota_sienna_manual.php