This could be caused by a number of stuff, most possible reason would be doing something contrary to Fiverr’s terms of service. Usually, once the Fiverr bot suspects something contrary to Fiverr TOS, it flags the account and this is usually an automatic action that will be reviewed by Fiver Customer Support themselves after a little while. If however you require a quick solution, you can either contact Fiverr support on the matter or find a “Not Spam” button on your conversation page with your client (May be there, may not be there, it’s all a matter of the reason for the temporary restriction placed on that person’s account)
I have faced this similar issue a number of times, all I did was click “Not Spam” on the page as this usually occurs on first contact from a prospective buyer seeking to know if I am able to answer his needs, on other occasions, I have fallen victim as I would get this message “Your message was not sent to bla bla bla, this could be as a result of the Violation of Our Terms of Service, please wait while we review this issue” First time I got that, I was trying to make a delivery, and because I couldn’t afford to wait for long as I was not sure how long it would take, I had to message my client on the “Conversation page” and sent her a snapshot as well to explain what I was experiencing, then I proceeded to contact the customer support. It was fixed in a matter of 15 minutes and my account was back to normal.
Another time it happened, I just didn’t care as I was totally sure I did nothing wrong, I left the conversation page and totally forgot something like that happened, eventually I didn’t see the alert from Fiverr again which left me to presume they had reviewed the issue and seen I did nothing wrong.
In some other cases, it could be as a result of a glitch just as @phantompower had said. However, in my own situation, it seemed like my account was flagged by the system bot and the message read “could be as a result of a possible violation of TOS”
My two cent; it’s always nothing to be too weary of, just tender your matter to CS as long as you are sure you have not in anyway violated their terms of service and viola, you are back to seamless operation!!!