POS 2.0 Database Connection has been lost......

At closing, while trying to settle the credit card transactions, we get this message: "The database connection has been lost. When you have resolved the problem, click Reconnect. If successful, POS will reopen, but unsaved work will be lost." This has happened a couple of times in a row and now we find out that customers have been charged multiple times ! The only references I could find in this forum tell me to take my debit card pad off and to restart the SQL service. This hasn't helped, still cannot settle. Anybody any pointers ? Thx