Follow the steps in your contract agreement, if you have one.
Make it clear you intend to recover the amount owing by whatever means available to you.
This might include penalties for late payment such as applying interest to the fee.
If your client is suffering genuine hardship you might offer payment terms so that you can at least start to recover the debt. For example, you might accept half now and the balance in a months time.
Where clients don't seem willing to negotiate or communicate, then you might consider legal action and this can be straightforward using a small claim tribunal where the amount owed fits within the small claim tribunal limit.
Another successful tactic I have used is to threaten their credit rating. As a sole operator freelancer in Australia, I can't report unpaid debts directly to credit agencies but I can hire a debt recovery company who can do this on my behalf. Clients will often respond positively when their access to credit is under threat. You don't usually have to hire a debt recovery company, but let the client know this is what you will do if you don't hear back from them with the next 48 hours or similar.