This actually happened to me the other day. I was told I would be paid to write multiple reviews on yelp in exchange for cash payment, however after I had written the reviews and showed proof of this to my client, they ghosted me and never responded with my payment. I was very taken aback as this was the first time something like this has happened to me... I emailed them back at first asking what was wrong, and then after one day, I sent them a very terse email demanding I be paid for the work I had done. Sadly there was no response and I ended up just eating my losses. There is really not much you can do if you are working for your client remotely, but if you have met them face to face, I recommend setting up an in-person meeting or even giving them a phone call to get to the bottom of the issue. Good Luck!!!
It's a common problem and there are a number of solutions. In this situation, you can tell them that you will write negative reviews and this will trigger other reviewers to do the same. In some countries like the UK, it is enough to issue an invoice and ask the client to acknowledge its receipt via email before you start working. Then, it is a simple process to address the claim via the Small Claims Court. Most companies fear that their credit rating will be negatively impacted and settle in no time.