If the terms of the invoice state 14 days and it is not paid within that time, then you need to start debt recovery. This varies according to where you live.
In the UK, you should now send your client a notice informing them that their invoice is overdue asking them to pay immediately. You should state that you will use the court service to obtain the money owed if it is not forthcoming.
You then follow up with another invoice, typically another invoice period later, which includes interest and debt recovery fees. In the UK these are provided for by law (at preset rates) unless otherwise specified in your invoice or contract. Always mark these repeated invoices for immediate payment, overdue.
Whilst you use other solutions to get your money, always keep sending revised invoices at your regular invoicing interval, with increased interest and recovery charges as permitted.
Once the first repeated invoice and notice has been ignored, then start a money claim through the small claims court. This is generally inexpensive, and legal fees are low as you (at least in the UK) have to represent yourself. If the person who owes you money fails to turn up, then you automatically win. The losing party pays the court costs.
Finally, speak to a lawyer about declaring the other party bankrupt should they fail to be able to pay you.