If I just found out my husband divorced me in 2013 after being married 9 years, how do I get my 50% share of his 401k?

You can try get the Decree set aside on the basis of fraud, but much more will need to be known as to how this happened and what is showing in the paperwork. Get the paperwork from the court and have it reviewed by an attorney.

While there are a lot of question to be asked, your first step should be a consultation with a qualified family law specialist to find out exactly how and why a divorce was processed without your knowledge; options, depending on what happened and what the decree says, involve a motion to set aside, partition of omitted assets, or submission of an enforcement QDRO. Time is not your friend - you should proceed as expeditiously as possible.