I have been in this situation and luckily managed to get all the money back in the end. Often, clients go a little shady or close-down comms with you when they're a little brassic. Not great for you, but unforytunately it happens from time to time. Have you firstly sent an invoice?
If not, then first things first, send one and outline your payment terms in that!
If you've sent the invoice, a polite email reminder to say that you've finished the agreed work, please let me know if you need another copy of the invoice so we can settle the bill. Absolutely do not do any more work for this client until it's settled (if there's any on-going work), and state that in the email. Offer them a get-out i.e. if you'd like to discuss alternative payment arrangements (two or three separate payments rather than one for example). If the client has an accounts dept. it may just be that they're not the people to be billing and as such, the invoice has just got lost in the business somewhere. Contact the company and ask for the accounts payable department or at least where to send invoices to.
In UK law we can charge an 8% interest rate on late payments but you will need to have stated this in the original agreements with your client. (Nice to know for the future though, as it can help!)
If you get no response in 3 working days (that's how long I think is fair) PHONE THEM and just ask if they've received your emails. Sometimes they just need a gentle nudge!
If after the payment date comes and goes, and you've heard nothing (and you've done all of what I suggest), you have a couple of choices: 1. Write it off if it's not that much money or 2. Call in a collection agency - they're not that expensive and although you'll have to give them a payment, so only worthwhile if it's a decent amount you're looking for.
In future (hoping that they pay), always, ALWAYS, always get a deposit from this client BEFORE starting any work.
HTH and good luck!
Gem x