A B737 CREW RPTS DIFFICULTY IDENTIFYING A CABIN ALT WARNING HORN WITH NO SECONDARY INDICATIONS FOLLOWING A BLEED VALVE FAILURE AND LOSS OF CABIN PRESSURIZATION.

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UPON ARRIVAL AT ACFT; I NOTICED THAT PRIOR CREW LEFT BOTH IGNITERS (RT SELECTED) ON AND BOTH ENGINE ANTI-ICE ON. I SWITCHED THEM ALL OFF AND PROCEEDED WITH A NORMAL PREFLT. UNEVENTFUL START UP; TAXI; TAKEOFF; CLBOUT; AND LEVELOFF. CAPT CHKED ACFT PRESSURIZING THROUGH FL180. FL350 ABOUT A THIRD OF WAY TO DEST; IN CRUISE WITH AUTOPLT B COUPLED; INFLT WARNING HORN BEGINS TO BLEEP WITH NO OTHER INDICATIONS AT ALL. RECALLED; BUT NO INDICATIONS; NO WARNING LIGHT INDICATIONS ON THE PRESSURIZATION PANEL. AFTER A FEW SECONDS; CAPT NOTICED THE CABIN PRESSURE CLBING AND OVER 10000 FT. WE DONNED O2 MASKS; ESTABLISHED COM; CLICKED AUTOPLT OFF (CAPT AT CTLS; I TALKED TO ATC); AND BEGAN DSCNT WITH CTR CONCURRENCE. RAN APPROPRIATE QRC CHKLIST TO COMPLETION. IN STANDBY MODE; THE ACFT BEGAN TO PRESSURIZATION NORMALLY. WE STOPPED DSCNT AT FL240 AND REMOVED O2 MASKS. CAPT TALKED TO DISPATCH AND MAINT. IT WAS AGREED UPON THAT WE COULD CONTINUE TO ZZZ WITH THE STANDBY SYSTEM WORKING AND THE MANUAL PRESSURE SYSTEM WORKING (CHKED GOOD ON PRE-FLT). CLBED BACK TO FL310; BRIEFED FLT ATTENDANTS; AND PAX. AT NO TIME DID THE 'AUTO FAIL' LIGHT ILLUMINATE ON THE PRESSURIZATION PANEL. THE PRESSURE SYSTEM DID NOT AUTOMATICALLY SWITCH OVER TO THE STANDBY SYSTEM; IT WAS MANUALLY SELECTED. AFTER LEVELOFF AT FL310 AND EVERYTHING ELSE HANDLED; WE BEGAN DISCUSSING WHAT HAPPENED AND WHAT THE CAUSE MAY HAVE BEEN. AT THIS TIME (NEAR DSCNT POINT INTO ZZZ); I NOTICED THAT THE L BLEED AIR PSI WAS ONLY AT APPROX 6 PSI. R BLEED WAS NORMAL AND PROPERLY PRESSURIZING/HEATING THE ACFT. THIS INFO WAS SENT TO MAINT VIA ACARS. DURING DSCNT ABOUT FL180; I CONTINUED TO LOOK AROUND THE COCKPIT. KNOWING THAT THE PACKS/BLEED CIRCUIT BREAKERS WERE BEHIND ME; I LOOKED. A SINGLE CIRCUIT BREAKER WAS POPPED. IT WAS THE 'AIR CONDITIONING; BLEED AIR VALVES; L CIRCUIT BREAKER. CONTINUED TO UNEVENTFUL VISUAL TO ZZZ RWY 2. CAPT SPOKE TO MAINT AND DISPATCH ON THE GND. THEY DEFERRED THE PRESSURE PROB AND RESET THE CIRCUIT BREAKER. WHILE WAITING FOR THE DELAYED WHEELS UP TIME; MAINT CALLED THE JETWAY PHONE. I ANSWERED AS THE CAPT WAS AWAY. NEW MAINT REP HAD READ NOTES OF DEPARTING MAINT REP AND WAS 'CONFUSED' BY THE WRITE UP/DEFERRAL. I REITERATED THE EVENTS/INDICATIONS STEP BY STEP; AS WELL AS CONFIRMED WHICH CIRCUIT BREAKER WAS POPPED. HE STATED THAT THE WRONG DEFERRAL WAS MADE FOR A BAD L PACK CIRCUIT BREAKER. CAPT CAME BACK AND SPOKE TO HIM AS WELL. THE CONCLUSION WAS THAT THERE WAS PROBABLY NO BLEED AIR BEING PRODUCED BY THE L ENGINE DUE TO THE CIRCUIT BREAKER BRING POPPED. HENCE; NO BLEED AIR FOR THE L ENGINE ANTI-ICE. ZZZ1 HAD RIME ICING BETWEEN 1000 AND 8000 FT. THE ENGINE NEEDED TO BE RUN AND THE BLEED SYSTEM CHKED BY AUTHORIZED MECHANIC. CONTRACT MAINT ARRIVED; PLUGGED-IN AND HAD US START THE L ENGINE. NORMAL START; NORMAL ENGINE BLEED PSI PRODUCTION. HE HAD US 'STRESS' THE BLEED SYSTEM BY RUNNING PACKS IN HIGH; CLOSE CROSS BLEEDS; ETC... I TURNED ON THE L ENGINE ANTI-ICE FOR MY OWN EDIFICATION. IT WORKED NORMALLY PER COCKPIT INDICATIONS. TURNED THE ANTI-ICE OFF. I THEN LOOKED AT THE CIRCUIT BREAKER IN QUESTION (AIR CONDITIONING; BLEED AIR VALVE; L). IT HAD POPPED OUT AT SOME POINT DURING THE ENGINE RUN; I INFORMED THE CAPT; WHO INFORMED THE HOOKED UP CONTRACT MECHANIC. WE SHUT THE ENGINE DOWN. THE END RESULT WAS THAT THIS COULD NOT BE DEFERRED; THE FLT WAS CANCELLED.