Yes, flip the left and right channels from your turntable to the preamp. If the channel out remains, then it is the preamp. If the channel out switches, then the table is the source.
If the table is the source, unscrew the headshell and take a look at the connectors on the phono cartridge pins. Did one come off, or are any loose? You could also take a volt/ohm meter and check for continuity from cartridge pin to headshell connector.
The other area of trouble for tables is the interconnects and more then likely, the molded on RCA connectors can be suspect. To prove that, take your volt/ohm meter from the connector on the arm (with headshell removed) to the the RCA jacks. There are 4 pads at the end of the arm that eventually get connected to the shield or tip of the RCA jack, just can't tell you which is which, but eventually thru trial and error you should have continuity from all 4 pads.
If you have determined that one of the RCA jacks is bad, you will have to replace them.
Wayner