I had a heifer to prolapse after a hard birth about three weeks ago. Using the truck headlights since it was about 2 am in the morning, we put her in the headgate to stuff it back in. My dad stood across the top of the chute and we placed a towel under the exposed part. He then picked everything up so that all I had to do was push horizontally, as opposed to up and in at the same time. Everything went back in rather easily using this method. We also used sugar. After everything was back in, we put three hog rings on each side and laced her up just like a work boot. Removed everything a few days later. Gave her a few rounds of antibiotics also.
As Dun stated, if it has been exposed to air very long it may have started to dry out to the point it cannot be put back in. We had that happen one time and the vet tied it off best he could up close to the cow and removed it. He didn't give her very good chances but she did survive and raised her calf.