There are various ways to attack it.
My first approach is to bash the flange on the tool holder with a hard plastic hammer although a lead hammer would be okay. Just bang from different directions and then try a manual tool change.
If that doesn't work I go to a piece of aluminum bar as a drift and a heavy hammer and bash away. The idea of using something soft is to avoid damaging the holder.
If that doesn't work you curse a bit then get someone with a pry bar or very large screwdriver......But first put a lot of something soft on the table and lower the spindle so the holder does not have far to drop.
Have your helper wedge the prybar in to be pulling the tool holder out while you press the release button.
If none of these work you really need to curse a lot; I have run out of ideas.
To reduce the chance of it happening again make sure both the spindle and holder tapers are spotlessly clean. Get off all the residue that builds up from oil and coolant. Some people use Scotchbrite but I prefer cloth and use both methanol and varsol separately to dissolve the different types of residue.
Then lubricate them with very light machine oil.
Oftem the reason the holders stick is a combination of the residue that builds up, running the tool very hard to warm up the spindle and then leaving that tool in the spindle as it cools down.