I have a question or two.
Did you know there was a 401k plan? If so, did you ask about enrollment? Have you carefully gone over all the paperwork you were given at hire and determined that there was NO reference to it?
In other words, is your employer going to be able to pull out some form of paperwork that will show that you were given the information and simply did not read it? (If I had a dollar for every time an employee complained that they were not given this or that piece of information and I was able to reach into a desk drawer and pull out a piece of paper with their signature on it explaining the point they claimed they were never told, I could retire.)
You cannot sue, no. Dave's suggestion would be the way to go if you can confirm that you had NO possible way of knowing that this benefit existed and that you could participate. But before you go that way, I suggest you VERY carefully review your new hire paperwork, employee handbook, etc. because if your employer has that same desk drawer I have, you're sunk.