Assume it is the same as in the 105 DC ... This is locked by screwing the hood into very fine threads on the inside. These threads jam easily and the lens hood then is pretty much stuck. Very careful rotating the hood a little back and forth might make the threads engage again and you can undo the hood.
If the hood is struck completely, the front casing of the lens has to be removed so you can access the threads from the inside. This is awkward on the 105DC and probably the same on the 135 DC.
I have just left my 105 DC hood in the locked position and make no attempt to collapse it for transport.
