You have to read the code to find out. The flashing light means that you need to do a diagnosis. A common failure on cars like yours is the transmission control module, but you should also check your fluid levels of course.
The codes can be read through a data link connector that, when properly hooked up, will give you a series of flashing lights to read. These are called "manual flash codes", used by the older OBD-I systems.
A good workshop manual should tell you how to do this, there are a lot of details, or any good shop can read it for you.