Its prob that ingame bug thinggy again.... just open steam settings... ingame tab... enable steam community in-game tick it, un tick it.. tick it again and press ok.. now try ;)
Theres only one thing that will save L4D imo...
Release the SDK for L4D but DO NOT release the SDK for L4D2.
That way mod'ers can keep the L4D community going. I prefer Mods to the originals.
nyght