i recently had the same exact problem with my yj where the clutch was working just fine then out of the blue i couldnt get it into gear. I checked my brake fluid reservoir and it was almost empty. I stuck in some DOT3 brake fluid and then pumped the clutch a ton until it stiffened up and it's back to normal. i would definately refill your brake reservoir, pump the clutch until it's stiff and you should be good to go!
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Eric - *'92 White YJ, 4.5" custom Deaver springs, 35" MTRs, Tera-Low 4:1, Hydro Assist, CAI, Rear 8.8, Chromo D30, 4.88s, locked front/rear, full cage, tube fenders.
*'08 Silver Rubicon JKU 2.5" lift, 5.13's and 35's