Yeah, I don't know what tank you have, and how badly it is "stuck."
For me, sometimes (and you'd have to be cautious, as it is attached to the battery, a "stuck tank" although it's usually removed from the mod, for me) can be run under warm water which sometimes loosens any eliquid leaking or filling issues that can kind of "fuse" the mod shut. You could also try the freezer (similar situation) although not for long you don't want to (again) damage your mod etc. I would start by using rubber gloves and a rubber mat to see if you generate enough grip and friction to loosen it.
Inserting something into the airholes to use as leverage can sometimes work, but use caution, you don't want to damage anything. Many vape shops will sell ceramic tweezers that come with a "grip your tank" feature (it's just like what they use for stubborn lids, only more tank sized) that can also increase your leverage.
The most important thing is to attack it not too hard, and possibly in stages, as each attempt will probably loosen it some and don't get over frustrated as you go so you don't damage your gear.
If you post what your tank is someone may have their own personal method that tends to be successful with that tank, so probably worth doing (if you haven't already fixed it).
Posting your "solution" on the thread can help others, too, who may be running into a similar issue and may happen across your thread.
Good luck, (unless you already don't need it).
Anna