I have o2 broadband and i am connecting through a wired connection, unable to negotiate a lease with DHCP?

Ii have o2 broadband and i am connecting through a wired connection, unable to negoiate a lease with DHCP server...what do i do now?? i got it connected before then it neva got connected again! im totally lost and pissed that its not working....i need help asap! PLEEASE!!