sify bb does not connect to connect

hi,can anybody help me out.

nowadays my sify bb does not connect to connect .whenever i try to connect to ,an error occurs saying that my ip address is
being used by another computer on the network and so the local area interface is disabled.i contacted the sify customer too,
but he denies that my ip address being given to other computer on the network.

i think somebody illegally works with my ip address and it is untraceable.please help me out to remove this problem.

the remot computer is very smart enough.