Originally Posted by mcf
Ahh, thanks. I was hoping there was a secret little switch rather than fear-inducing twisting. Will give it another go.
Drip a few drops of 97% alcohol into the groove between hood and thread and let it sit for half an hour. Also, before trying to unscrew again, carefully but firmly tap the hood all around the front edge while setting it with the rear cap onto a firm surface.
The alcohol might dissolve anything chemical that's in there and bonds the parts, the tapping might loosen any oxidation between the thread and the (probably) aluminium lens hood.
Let us know what did the trick!?