FLT CREW OF A CARJ FAIL TO REPROGRAM FMC FOR RWY CHANGE ON DEP FROM ATL. RECOVER FROM TRACK DEV WITH ASSISTANCE OF DEP CTLR.

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PRIOR TO PUSHBACK WE RECEIVED CLRNC FOR RWY 8R AND THE SUMMT ONE RNAV DEP. ON TAXI OUT WE RECEIVED INSTRUCTION TO TAXI TO RWY 26L. BOTH THE FO AND MYSELF; WHILE PERFORMING THE 'TKOF BRIEF;' OVERLOOKED THE RWY CHANGE. TOGA WAS SELECTED ONCE CLRED FOR TKOF; UPDATING THE FMS POSITION. ON ROTATION THE NEW PROC WAS IMPLEMENTED. 'GEAR UP; SPD MODE; NAV MODE;' WAS CALLED FOR. THE FLT DIRECTOR STARTED A SLOW TURN TO THE L. I MOMENTARILY FOLLOWED IT; NOTICING THAT THERE WAS NO INFO ON MY MFD. I CORRECTED MY HDG TO THE INITIAL DEP PROC HDG. THEN I IMMEDIATELY NOTIFIED THE TWR THAT WE HAD LOST NAVS AND WOULD NEED VECTORS. A 275/280 DEG HDG WAS GIVEN THEN WE WERE HANDED OFF TO DEP. AFTER CHKING IN WITH DEP; WE WERE ASKED IF WE COULD CONTINUE. WE RESPONDED THAT WE THOUGHT THAT WE COULD. DEP THEN GAVE US A TURN TO THE N. WITH THE AUTOPLT ENGAGED AND THE AFTER TKOF CHK COMPLETE; I REENTERED THE RWY AND SEQUENCED THE FIXES. I NOTICED THAT WE HAD NOT STRAYED VERY FAR FROM THE DEP. WE INFORMED ATC THAT WE HAD NAVIGATION CAPABILITY AGAIN. WE WERE CLRED ON COURSE. NO FURTHER EVENTS OUT OF THE NORMAL PROCS OCCURRED. NEITHER I NOR THE FO KNEW THAT THE LOSS OF NAVIGATION WAS DUE TO NOT UPDATING THE FMS AFTER A RWY CHANGE AND NOT CATCHING THE ERROR BEFORE TKOF. WE BELIEVE THAT THIS WAS THE CASE AFTER CAREFUL DISCUSSION. THIS WAS THE FOURTH DAY FLYING WITH A FAIRLY NEW FO WITH ONLY A FEW MONTHS ON LINE. WE WERE FATIGUED AND I BELIEVE THAT TO BE A STRONG CONTRIBUTING FACTOR.