Cisco IOS problem

Hello,
im pretty a noob concerning Cisco IOS but perhaps someone here can help me with my problem.
We got an Cisco 851 ADSL Router which has the IP 192.168.168.1 in the internal network. There was supposed to be a server with the IP 192.168.168.104 also in the internal network. Yesterday i tried to forward a port to this server so that i would be able to connect to it via remote desktop. Somehow i got it all wrong and now the IP is reserved and i had to give the server a new ip what created new problems. Can somebody tell me how to figure out what network device is reserving the IP (192.168.168.104) and make it release it?

greets,
Adel