OK - so I got the elder scroll but I've left the cube on the machine! I realised a bit late, fast travelled back but I can't seem to get back in....the gate's locked! Any other way in? AAARGH!!!
I don't think you need the Lexicon, do you?
I think you'll need to either 1) reload a save prior to the gate being locked, 2) try find a way around the door, or 3), forget about it and do it on another playthrough. If I remember correctly, it's part of Hermaeus' Daedric Quest but not the main quest?
They want to see me shot up, locked up, encage me...
I come back bigger... stronger... and angrier.
I thought about going to an earlier save but I've played about two hours before I realised my mistake...sob....
You should be able to get back in the way you got there through Blackreach. Did that not work? You don't need the Lexicon except to finish the Daedric quest which started when you talked to the Abyss. You should always go unlock the gates into Blackreach from the surface.
I am the cat who walks by herself
As I recall, although there are several entrance/exit gates to/from Blackreach, only one-at-a-time is unlocked. It should be the same gate that you last exited Blackreach from when you returned to Skyrim. After you got the Elder Scroll, did you trudge all the way back through Blackreach or did you take that nice little shortcut? Whichever gate you used to leave, that's your only return path. And, Yes, you do need the inscribed Lexicon to complete the Oblivion Walker (daedric artifacts) achievement, and you need those special nimroots to complete a side quest, so it is worth returning.
If we knew what we were doing, it wouldn't be called research.
I spent about an hour and a half, real time, wandering around in the dark in Blackreach, searching for the Red Ninroots... even though they chime when you are close, you have to be practically standing on them to hear it... I read in the Skyrim Wiki that they grow mostly near the large gellyfish and near water, so that helped narrow down the search... the reward for that side quest wasn't worth it, inho, though Blackreach itself is amazing... the size of it alone would have warrented it being DLC, but they put it in the game from the start... the DLC will be amaing, I am sure...
There are literally 4 ways into BlackReach that I know of. There is one way in if you have the key and leave the gate open. You should be able to get in from, I think it's Saarthal, anytime you want once you do that quest but it's a long way. I want to say the other one is top of the mountain near WindHelm. The other hard way is through the Copper Cave or something like that also near Windhelm. I never heard you can only do one at a time. Everytime I wanted to go back down I just went to the one I was closest to. Now all this was done AFTER I used those same places as exits so they were always left open when I left and I always found them still open when I got back. It is entirely possible that when you left, you exited from a different gate. Try to retrace your original entry as that should still be available to you.
...Angelus ignem tenebris blue... Semper Fi! Semper Cautus!
Retired Sailor, growing up backwards! Not here to offend anyone!