You need to get it sorted as soon as possible.
When mine went tight the thrust bearing overheated and melted!
Start by squirting penetrating oil where the shaft enters the clutch housing.
Do this untill the arm feels free to move by hand. Disconnect the clutch cylinder to make sure arm is free to move, but re-connect to work in new oil Access through the top, or through the nearside wheel arch after removing the wheel. Use good oil, not WD40!
Keep working the clutch after each liberal spraying.
This should eventually work the arm loose.
Then drop in some fine lubricating oil, such as 3-in-one, again work it in.
Finally use lithium grease spray and try and work that in. Clean up any excess when finished.
Keep an eye on it for a few months to make sure it does not tighten back up, and repeat above if it does.
Then every six months or so, re-lube it with spray lithium grease and clean away excess.
Alternative is to remove gearbox and remove arm and replace with modified arm, or clean up and grease old arm.