A319 FLT CREW HAS STAR DROP OUT OF THE MCDU WHEN GIVEN A RWY CHANGE DURING ARR TO LAX AND IS WITHOUT RNAV CAPABILITY.

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WE WERE JUST ABOUT TO HIT THE FIRST FIX ON A STAR INTO LAX WHEN ATC CHANGED OUR LNDG RWY. IN THE BOXES ABOVE; I PUT CIVET ARR; BUT I CAN'T RECALL THE ACTUAL ARR. REGARDLESS; THE CAPT QUICKLY ATTEMPTED TO RELOAD THE MCDU WITH THE RWY CHANGE BUT THAT DUMPED THE REST OF THE STAR LEAVING ME WITH ONLY 1 FIX ABOUT 2 MI AHEAD AND A DISCONTINUITY BEYOND THAT. I ADVISED THE CAPT TO INSTALL THE NEXT FEW FIXES ON THE STAR ASAP SO I'D HAVE SOMETHING UPON WHICH TO NAV. FOR PART OF THE ARR; WE WERE WITHOUT FIXES AND APPROXIMATING THE COURSE WE WERE TO FLY BECAUSE I HAD NOTHING ELSE AVAILABLE. I ASKED THE CAPT TO ADVISE ATC WE WERE HAVING DIFFICULTY AND NEEDED DIRECT A POINT DOWNLINE THAT WE COULD INTERCEPT THE STAR/APCH. EVENTUALLY HE GOT SOMETHING BY WHICH I COULD NAV BUT WHAT CONCERNS ME IS THAT GAP WHEN WE HAD NOTHING AND COULD POSSIBLY HAVE BEEN OFF COURSE OR EVEN ALT SINCE THERE ARE MANY STEP-DOWN ALTS ON THE STAR. ALTHOUGH IT'S UNLIKELY THAT WE WERE SIGNIFICANTLY OFF COURSE AND LESS LIKELY WE WERE OFF ALT; WE SHOULD HAVE NOTIFIED ATC IMMEDIATELY RATHER THAN RISK IT.