Assuming it's a manual then I would say that there is a problem with the clutch. Probably the pivot has pushed thru the release arm.
is this exspensive
could there be any other possible causes
i had the slve and master cylinders off as i was told one or the other could be the problem master was a bit gunky so cleaned it out and replaced think i blead it properly could air in system be a problem
i took the master out and there was gunk in there like black dirty fluid at the bottom cleaned it out with new fluid and replaced it the problem is still as was when first had problem no gears but as i said if i select first when engine off then turn on it will pull away ok. also had slave out checked push fork and rod both fine and slave working fine im totaly stumped as to what to do next
can you describe what you mean by it wont change gears.. how does the clutch feel, what stops it changing gear.. can you take it out of first but not get it into another gear etc.. seems so far peeps are trying to help you but your not helping them much...
If the fork is ok then you have air in the system. You need to bleed the clutch system. Did you bleed it after re-fitting the master cylinder?