Why are you trying to change your IP address?
By default its set to dynamic (automatic) Did you change it to static yourself?
Are you sure you're assigning an address appropriate for the subnet mask being used?
Is it possible to change a static IP without contacting the ISP
If the IP address you are trying to change is "private", then you CAN change it, but you have no control over the "public" IP address assigned to you by your ISP..