Can no longer shift to lowest gear I recently worked on my derailleur a bit to fix some issues in high gears, however, now I can no longer shift down to my lowest in the cassette when in either crankset.
I have tried the L screw at about any setting, including turning it so far it came completely out, but am still unable to downshift to the largest cog.
I also turned the barrel adjuster quite a bit, with no noticeable difference that I could detect. (Is it possible I need to do many, many turns to get it where it needs to be? It seems unlikely considering I didn't have to make massive changes before to get the other cogs set, but I admit I'm a novice.)
I'm thinking either the spring itself is bad or maybe I need to tweak the B-screw, but I'm afraid of undoing my work that got the higher gears working and going back to square one, or worse, with my shifting. Any help is much appreciated. Thanks in advance!
This sounds like a situation where you should have a professional bike mechanic look at the problem.  If you have attempted to resolve the problem by adjusting both the correct limit screw (you did make sure you were adjusting the correct limit screw, right?) and the indexing via the barrel adjuster, then something else is wrong.  One possibility is that your derailleur hanger is bent, although without seeing it I could not say that is the problem for sure.  Whatever the issue is, if you can't find it yourself, you'd be best served by pulling in some additional help rather than floundering and potentially throwing everything else out of whack.