Hi All: I have a `87 420 SEL. While parking the car, the trans. "magically" went into neutral in every range! There wasn't any prior problems/warnings. What's going on???

I removed the trans., replaced all the seals, etc., the reverse clutch lined plates, re-installed the trans......all gears still neutral!! What is the problem???? HELP!!!!

Thanx in advance.......
Glenn