It may be down to conflicts relating to the DHCP server which dynamically allocates IP addresses.
In your scenario it should be the router.
Bring up a command prompt and issue the command IPCONFIG /ALL then check the address of the gateway which should be something like 192.168.0.1 which should be the address of the router. Check the address of the DHCP server which should also be 192.168.0.1. Do this both when you have access and when you dont. If you come across an address beginning 168 then this means your PC cannot access the DHCP server and has assigned one of its own.
It could be that there is more than one DHCP server or that restrictions are in place on the router.
Unfortunately as you cannot access the router then you cant check this out.
Hope this helps.