not-logged off users... how to decide whether a user is connected or not?

As it happens, (no idea why), a user is disconnceted after a poweroutage/powersurge/modem disconnection or whatever the reason is but the radius doesnt see the user as online. The record in the radacct table has a Start Time but doesnt have a Stop time, the AcctStopTime is set at " 0000-00-00 00:00:00" which is probably because the user was disconnected abruptly and didn't have the chance to send the Account Stop packet or a Disconnect packet. Anyway, to the bottom line - how do i deal with such situation? Thanks in advance.