I had a 401k plan at a job I worked back in 1983-1988. I left that job, and don't know whatever happened to the money I invested.

It should be still with the plan's administrator. It is possible, however, that it has been sent to the state as unclaimed funds.

You need to contact the company you worked for or the plan administrator. Also, they are required to notify the SSA that you have a retirement plan so when you file for SS they will inform you of any monies you have.