There are various reasons why Reverb would suspend your account or place it under review. The most common reason is that you've got an unpaid Reverb statement, and it is past the 15th of the month.
Other reasons include multiple instances of unacceptable behavior, not fulfilling orders, etc.
Reverb Support is unable to discuss any account statuses on live support, so make sure to reply back directly to the email you received from Reverb regarding the review.
If you did not receive an email from Reverb Support, please fill out a support ticket for further assistance.