As a dairy farmer in Illinois, I can assure you that hooking a calve up to a tractor and pulling it is probably the most risky way to accomplish the calving process. There are a few stages that take place during calving and it can appear that a first calf heifer probably was tired of the calving or that she was simply taking a break from the process. I have seen many times where a cow or heifer will take several breaks from calving, and in some cases will actually eat or drink while calving. Did the cow look distressed, was she laying down or standing up? Let's face it, cows have been having calves all by themselves for thousands of years, and the first thing people assume is that you MUST pull that calf. We will generally let the cow take her own course. Most people pull the calves as soon as hooves are showing, and thats not usually a good thing. She must be dialiated far enough to physically expel the calf from her opening. Many future problems occur with breeding from pulling calves way to early. You can certainly rip the cows vulva and cause tremendous internal damage also, but lets not forget about the trauma to the calf. Maximum pressure to pull a calf is somewhere in the neighborhood of 150-185 lbs of pressure, never more than 2 adults can physically pull is a good rule of thumb for the max. Breaking the legs of the calf or causing nervous system damage is not uncommon. In your case, if no other help was available, by all means- call a Veterinarian. He or she may have determined that the heifer was not dialated enough to properly calve and in the worst case would have C-Sectioned that baby out. ALWAYS palpate the cow to confirm the position of the calf, is it head first or backwards. Never assume that if you see to hooves coming that the head is there also. Many times they can be head back and coming out in which case is a "so to speak" simple fix by repositioning the head forward, and then allow the cow to continue the course. If no process is made within a half hour, you could give the cow a dose of Oxytocin, usually 10CC or so to allow her to dialate, and more importantly RELAX. Cows like to be alone and relaxed during calving so they can concentrate, yes, concentrate on calving. Bottom line, NEVER force a cow to calve beyond what she cannot physically accomplish herself. If YOU and the COW both relax, and not get into a hurry, you will find that many cows can and will calve all by themselves. Some cows take up to 4-5 hours to calve, and that's ok provided the calf is in the correct position within the birthing canal. If you can see a tongue, pinch it and see if the tongue returns to a normal color or whether it stays a blue-purple color, this a sign of distress. Pulling a calf with a truck or tractor is a sure way of causing more problems than you beg for. You can lose the calf, cow, or both in some cases. You're young and have a lot to learn, I just hope you learn a little from this reply. The biggest thing is to be patient and relax. Cows are very good and sensing anxiety within people, and she is more likely to tense up and go the opposite direction in which you are trying to accomplish.