If the fans don't come on chances are that you're refrigerant has leaked out. The AC system will shut down to protect the compressor from burning out. On my 98, I had it professionally charge with a tracer die. After 2 days, I could see that the rear AC lines were leaking. After trying to repairing it and throwing money away and learning about incompetent mechanics, my final option was to cap off the rear lines and no longer have rear AC.
Toyota dealers will not do this type of repair. I have details at the following link.
http://www.siennachat.com/forum/81-problems-issues/10058-98-sienna-finding-ac-leak.html
Hope this helps. I would get it charged up to find the leak first and then go from there if I were you. Rear lines and repair could cost you $1000 easily. Blocking off the rear lines is less than $100 and then the cost to recharge the system. The van cools off fine with only the front AC unit working. You may need to set the fans on max, until its cool
Regards, JC.