One first troubleshooting step would be to try running tabcmd from the actual 7.0 bin directory. If this does not work, I would recommend sending this in to our support team (you can fill out a form at http://www.tableausoftware.com/support/request) with the following information:
1. Date & Time of last successful refresh.
2. Date & Time of last unsuccessful refresh.
3. Screen shots of error message
4. Log files (See http://www.tableausoftware.com/support/knowledge-base/creating-tableau-server-log-files for instructions on bundling and sending the log files).