Mazda 3 Won't shift!

I have a 20004 Mazda 3 that will not shit into gear. Had this problem about a month ago. The car would not go more than 10 MPH and it made this wining sound at about 10 MPH. Took it to the mechanic and he replaced some sort of sensor and that seemed to have fixed the problem up until a day ago and it started to do it again.

Does anyone know what this sensor is the he replaced and why it may be doing this again, after he supposedly fixed it. Please help!

Thanks Joe...........