Motorcycle Repair: yz 125 wont shift, motorcycle clutches, yz 125

Question
ok i have a yz 125 2000 and it all the sudden won't shift. i changed the oil and it worked for a bit but the gears were off and out of no where the shifter is just stuck after riding for a few minutes. I am pretty sure i need new clutch plates cause it won't move freely when its engaged. it didn't shift very well ever since i got my new top end put on.  I started taking the right cover off but i just don't have time to get to it.. i hope its simple... what are some symptoms of a transmission seizing? cause my bike ran fine shifted well then my clutch was messed up for a few days and out of no where my gears where missing and minutes later i couldn't shift anymore and the shifter will only move like a few mm's up or down about the same distance both ways. i think its stuck in 1st but not sure which was my last gear till shutting it off. i am kind of hoping there is just something stuck in it like a piece of the clutch plate or something easy. but whats easy? I think there is synthetic oil in it at the moment. if i drain that and put yamalube should it loosen up? it said to put 10w 30 in.. and some one told me to do a synthetic 10w 30 so i did.  Any advice for what to look for it would be much appreciated.. i might just blow it up.   The shifter is just stuck and i hope its not life threatening to the bike or my wallet. thanks

Answer
Hi Tom,

The best investment you can make into your YZ, your wallet, and your safety, is to buy the service (repair) manual.

The manual has illustrated procedures for maintenance and repairs, and important information to prevent serious engine damages.

I suspect the shifter linkage is broken or out of place. Disassembly is regired to learn the actual cause of the problem(s).

USe sythetic oils designed for use with motorcycle clutches. Otherwise, regular synth oils may cause clutch plates to not work properly.

Respectfully,
Mark Shively