Assuming you are a participant in the plan, you need to immediately contact the trustee of the plan. In simple terms, the trustee is the person/company that is responsible for holding and safe-guarding the plan's money (i.e., making sure the employer doesn't use the money themselves). If there is a corporate trustee (a bank for example), then you should eventually receive your distribution. However, in the worst case scenario, the trustee could be an officer of the employer, in which case your money might disappear.
If you can't contact the trustee, I would immediately call the Department of Labor. Before you contact the DOL, you should find out as much information about the plan as you can (official name of the plan, employer's identification number, plan number, trustee, etc.). Try to find a copy of the summary plan description. If you can't find an SPD, go to
www.freeerisa.com and pull up the Form 5500 that was filed for your plan.
Good luck.