The most common reason is an unstable internet connection. This is especially true in wireless networks, which may experience momentary interruptions.
It's advisable to wait about half an hour first and check if your buy-in has been returned to your gaming account.
If the problems persist even with a wired internet connection, please contact customer service. In that case, also send the hand number and the time you were logged out of the table.
If customer service is closed, send an email to customer service at support@paf.com.