Assuming you are in England and Wales, it depends on whether the lease has been contracted out of the Landlord and Tenant Act 1954. If it has been contracted out of the Act this should be mentioned somewhere in the lease. If it has not been contracted out, the lease will continue effectively as a rolling contract after the lease expiry on the same terms until either the tenant serves three months notice to leave (a s27 notice) or either the landlord or tenant serves notice to bring the current lease to an end with proposals for a new one (s25 / s26 notice).