Basically this means that your computer's IP address lease has expired and the DHCP server (the router your behind) was not able to extend the lease or even issue a new one.
There are a couple of reasons that could be the cause which you will need to check out.
Do you have full connectivity to the network if its wireless or are all the cables securely connected and in good working order if your network is cable connected?
Have you upped the security on any firewall settings which may be blocking DHCP traffic or caused a change to DHCP server address?
Could also be a problem with your computers network card or driver, are you running the latest available drivers for it?
There are a couple of reasons that could be the cause which you will need to check out.
Do you have full connectivity to the network if its wireless or are all the cables securely connected and in good working order if your network is cable connected?
Have you upped the security on any firewall settings which may be blocking DHCP traffic or caused a change to DHCP server address?
Could also be a problem with your computers network card or driver, are you running the latest available drivers for it?