WD40 and a wirebrush (not to be confused with Wirebrush 'n dettol!)
Take care not to mangle your fingers - and as Myfeckin' pointed out - only adjust the height when there's no load on the unit!*
Once you get yer ring free'd up again you might want to give it a bit of grease to stop this from happening again..**
*No smutty comments here please..
**or here