A “contribution” to a 40l(k) plan must be made from your company paycheck.
Now, how did you get a $55,000 cashier’s check?? Just curious.
First thing to do is go to the bank and open a money market deposit account — nothing else! Deposit the check in there, and you will earn a very small amount of interest but have access to some or all of the money at any time.
Then think carefully about what you want to do with this windfall. First step — pay down debt!!