Transmission

I have an 03 Frontier with the 3.3 auto, 4wd with no problems at all. My distributor grenaded on me, now I've got the knock sensor code. The big prob is the transmission. It started not shifting right, the code reader said no 2nd gear... I had to drive it to work (babying it). I leave work and I have 2nd, however I lost 3 + 4!! One shop I brought it to said the trans is junk. How can you lose a gear than get it back and lose 2 more? Please Help!:banghead: