This has occasionally happened to me, it seems like the internal IP address changed via DHCP and the update didn't get reported.
You can try turning the media network on and off, which can regenerate a new Access Key.
Or you can try simply connecting JRemote to the server via IP address instead of access key.
One of those two methods should work for you, unless, as Jim pointed out, your configuration has changed and a firewall has started blocking stuff on your PC. You can verify that by pointing a browser or telnet session to the IP address on port 52199. For example:
http://192.168.1.10:523199 Yours,
-Paul