The first time I entered the Twilight Sepulchre I got killed hard and reverted to a save point just after I got done talking to Gallus but before entering where the stone door opens.
I left and did a ton of quests and leveled up but when I go back Gallus is still there and I have the journal but the stone door will not open to let me in.
Reloading is not an option as I've spent MANY hours in between.
I see no switch and my recollection is that the door opened by itself.
Can anyone offer any help?

I had this happen to be aswell I just made a new person and there isnt any way around it so you will have to go back to an old save or enjoy that last quest in your menu for awhile till they fix it.

While yes that svcks, but leaving in the middle of a quest is never a good idea. Especially when a door opens that you didn't open and you know you have to go through to finish the quest!

Next time you're stuck at a boss just turn down the difficulty,

Although in retrospect it may not have been a good idea to leave in the middle of a scripted event, even if I didn't know it was scripted at the time, the fact remains that Bethesda pushed the idea of coming back to a location that was too hard for you initially as a feature of the game.
How many times did we hear about caves/dungeons locking on your level the first time you enter it?
I'm just kind of bummed I have to start the whole thing over again with a different character.
On the bright side that means I get to play more Skyrim.

Same thing happened to me except that I had made it to the end. The quest would not update properly so I could not finish it and I reloaded a previous save and went and did other quests. When I came back to see if I could continue the door was closed and would not open.

[censored] im on the quest now, im happy i saw that because i prob would have left. thank you!