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);
}