I notice my skill for one handed has been taken back to 20 and shows in red- but can find no effects to account for it. tried a few potions and shrines anyway, no change.

any ideas?

Not sure but jail may affect it.

Its a bug with the game, happens with all skills sometimes afaik. When it happened to my lock pick, I just increased the skill by one level and it went back to where it should be.

Have never seen that in ~950 hours...

Well I've seen it once in 90

Happened to me once, in the Archery skill. Went back to normal after leveling the skill again some times - if leveling the skill once doesn't work, continue to play and to level, the game will eventually correct itself.

Notice that the skill may indicate "Level 20" in red, but while you navigate the skill perks tree, the real level is indicated, so no real implications on the game, just on the skills menu.

I got a console fix for you this happened to me as well....if you don't want to use it leveling it one notch will fix it and its purely cosmetic your stat value still should react correctly in game.

Here is the fix I copied from the S.T.E.P. guide so credit to them:
B2) Fix ingame stuck downgraded skill: -When a skill appears in red and it's much lower than it is supposed to be (and you got no negative magical effect active), then you have encountered a game glitch.
This is how to fix it: Once ingame, open console with the ~ or \ key.
Type: player.modav marksman 1 (enter) (replace "marksman" with the bugged skill).
Reopen the console. Type player.modav marksman -1 (enter) Your skill should be now fixed.

Note: my bug was one handed skill so the console was: onehanded NO SPACES FOR THE SKILL NAME