First, if you haven't already, call them and explain what happened: maybe there's an innocent explanation and they'll fix things (but even if the say they will, don't give them too long--just a week or two--before escalating). It's best to resolve matters without litigation when possible.If they won't fix it or won't even talk to you, your option is to sue them: you'd sue them for breach of contract (the agreement leading to your bonus, even if it was oral/unwritten, is still a contract). Be warned that if the company is insolvement, out of business, or bankrupt, you probably will not get the money: a bankruptcy would discarge the debt; and an insolvent or out of business company cannot pay, even if you get a court judgment in your favor.