If connecting to another router back-to-back, check if the interface cable is the DTE or DCE. If it is the DCE then use the interface command clock rate as one end needs to generate a clock otherwise the interface protocol does not come up. If it is connecting to your service provider then they should provide the clock and your interface is a DTE (so no clock rate command), but it does need the correct protocol (HDLC, PPP, Frame-Relay or whatever).