turn off physics.ignorecollision?

How do i do this if i've set something to ignore collision?

You need to pass "false" as the optional 3rd parameter (which means: stop ignoring collisions between these two objects).

For example:

function DisableForceField()
{
    Physics.IgnoreCollision( forcefield.collider, player.collider);
} 

function EnableForceField()
{
    Physics.IgnoreCollision( forcefield.collider, player.collider, false);
}