Problem: After logging into your FTP account you do not see any files displayed.
Solution: Login into Plesk and click on the 'Server' link on the left handside menu, then select 'Components Management' and make sure that the 'FTP Server' component is running.
Then click back on the 'Server' link on the left handside menu, then click on 'IP Addresses', then 'Firewall' and make sure the Firewall is running (this should be the case).
Also login to the server via Remote Desktop and click on 'Start' and then 'Run' and enter 'cmd'. From this black command prompt enter the command:
net stop ipnat
This should now of fixed the problem, but if this problem comes back after a server reboot, please disable the service 'ipnat' from the services. Ie Start>Run>services.msc