Would a faulty rear oxygen sensor be the cause of a jeep not starting?

What is your question?

My daughters 2000 Jeep Grand Cherokee went through some major repairs this summer. The last thing the shop said she had to replace was the rear oxygen sensor. They said it would run for a while but should be replaced/fixed. Of course this wasn't done and now the jeep won't start and is sitting in the college parking lot. Could that be the problem and if so, estimated cost of repair?