There are two ways, the second of which you will need when the first one doesn't help.
If the problem isn't too bad, using the lens a lot, giving the moving parts some exercise, helps work the grease into a better state (or rather: loose) and frees the lens.
The second one is to have the lens serviced, the old grease removed, the mechanics cleaned, and relubed.
If the first method works, it will still be necessary to send the lens in for a CLA sooner or later.
But if the focusing mount is so stiff that it will not move at all, there's no way you could get away using method 1. The problem may be more than just stiff grease. Metal chafings in the thread. Possibly a damaged thread too...
So i'm afraid it sound like your lens needs a repair shop.
It is not moving at all, so i think that grease is dead or gone, so i will try again with my hands if it can move even slightly but without rough so not going to damage it, if not moving then yes, send it to service.
Originally Posted by Q.G.
Thank you very much!
never force anything on a camera.
but you knew that
The 150 is the sharpest "C" lens I own at the moment.
It is also a non "T" version so if a deal comes on one they are generally a good bit cheaper.