2002 Toyota Echo fan blows but nothing happens

My sister and I were driving in very hot weather when her air conditioner stopped working. We could hear the fan blowing, and the air that did make it out of the vent was cool, but there was very little of it. For several hours, the air conditioner had operated perfectly. When the outside temperature went up to about 90, the cool air quit blowing out of the vents. If we turned the air conditioner off for a while (maybe 1/2 hour), we could turn it back on, and it would work again for a short time. The day before in 80 to 85 degree temps, it was fine. Any ideas? Thanks.