Sometimes this just happens. I haven't really found out why. But for the convenience of PC Linq when it works, it's worth sticking with it.
Try any or all of the following:
1. Make sure you have the latest version of PC-Linq running on both PCs. Click Help > About and it should say Version 1.64. If not, get the latest version via the link above.
2. Shutdown PC-Linq on both computers (if you have a task manager, use this to check that the application is truly shut down), unplug the cable, wait 10 seconds, plug the cable in again (using different USB sockets if available), Restart PC Linq.
3. As above, but shut down the PCs as well before restarting everything
4. Uninstall and reinstall PC-Linq on both computers.