Auto: 1984 380SE, 234000 miles, Auto 'box shifting problem.

Question: what can go wrong with the auto box - from one day to the next or over just a few (less than 100) miles - to make it so it doesn't shift properly / stay in first upto say, 40mph ? What circumstances could lead to this - very hard driving / abuse / whatever - any suggestions ? What effect does a low fluid level have on tranny operation ?

Any feedback greatly appreciated......

Simon.