Try spraying some PB Blaster on the shaft of the bit. Let it sit a few minutes and tap the the nut with your wrench. You may need to use a longer handled wrench or a cheater bar. I am assuming you left the bit in the collet for long enough for some nasty rust to seize it up. It's a good practice to remove the bit from the router when finished with the operation you needed it for.
If that doesn't work use a heat gun.
