A customer stopped paying the dues suddenly, what to do?

I own a small legal trading company in Dubai. One of our customers, whom we are supplying for over 20 years, has suddenly stopped paying us.

They're just delaying us whenever we call for payment and don't even meet us when we go there. We're on verge of trouble if we don't receive our payments.

We can't even afford to hire a lawyer. The amount is up to 400000 AED.

Please help us!