96 Lumina transmission problem

I recently brought my car into a mechanic to fix a problem with the steering, thye ended up replacing my power steering pump. The day I picked it up it had trouble switching gears and the check engine light came on. The next day it wouldn't go above 20 mph or reverse.

I brought it back to them and they told me the transmission went. Is it normal for a transmission to go that quickly without any warning signs prior? Should the mechanic have noticed something when diagnosising my car's problem when I originally brought it to them?

It does have roughly 180,000 miles but it has been very well maintained and most of the parts are new.