Huge problem, possibly transmisison

[Zooropa1310]

Well, it started yesterday. I was backing out of the gas station, and when I put the clutch back in, it was as if i slammed on the brakes. That only happened when I reversed, so I assumed something was wrong with the reverse gear. So I drive normally. I drive 50 miles on the highway, to meet my friend at a laundrymat. Afterwards, we pull out of the parking lot, I follow him, and I take the turn at about 10mph, switch into second gear, and my front tires lock up and my car stops completely, switching into neutral. I start it up, and cant shift. It made the most horrifying crack/crash I have ever heard. We had a apprentic mechanic with us, and he said it was the transmission, and the only way he pulled it to the side of the road was second gear, and it barely even did that. Can anyone tell me what happened? Also, price of repair estimate, high and low end. Would it be better to pick and pull a transmission and have it installed?

Some info:

135,000 miles
Sentra E 1.8L
5 speed manual
I've babied it for the last 5,000 miles (thats how long I've had it)
I've never driven recklessly with it, so I can't see how this is my fault.
Paid $1,000 for it from a reliable person, is it totalled?



Thanks