Hi,
If one of the caravan steadies (corner legs) won't wind down (or up) then the most likely cause is that the trunnion nut has broken up.
When you spin the shaft on the steady, a nut moves along a threaded shaft - forcing the leg down. Just like a car scissor jack.
These trunnion nuts can be replaced by a competent DIYer, local caravan shop should have replacements on hand - but there are two sizes - so you'll need to remove the nut before you go - and make an assessment of the condition of the other trunnion nuts - and buy a complete set of four more if you think these will also need replacing.
Alternatively you can replace the whole steady assembly, but this will be much more costly.
Robert