I started the Nightingale "Trinity Restored" quest, got my armor, listened to what Karliah had to say in the next chamber and agreed, then on our way to the ceremonial chamber Karliah turns around half way up the stairs and leaves without opening the gate. Any ideas as to why? I left and done like six other quest in the hopes that time was an issue--not so. Is this a bug? Or is there a quest I'm missing?

I have all the latest upgrades, patches and DLC's

Got a few mods but none that inhance or change anything related to the thiefs guild

I'm level 76 and have done a boat load of quest

Any insight would be greatly appreciated

I've done three ealier saves, no help--same results

reload an earlier save is your best bet

I haven't experienced this, so I don't know what would fix it, but uesp.net has a bunch of suggestions. Check out the fourth section in the Bugs list for the quest:

http://uesp.net/wiki/Skyrim:Trinity_Restored#Bugs