Again, thank you for all the valuable info.

I am going to buy SRAM X7 Type 2 10 speed.

SRAM X7 Type 2 10 Speed Rear Mech 2014 | Chain Reaction Cycles

My last question is how do I find out if i need medium cage or long cage? . I looked at the bikes spec and no info to be found.

I will install the mech and then take it to shop for them to make sure everything is ok. Cost of installing a mech here in UK would be more than the actual part

I was on downhill and I suddenly couldn't pedal anymore but the wheel was spinning fine so I guess it wasn't stuck in between wheel.