Irthe o kombos sto kteni

This literally means the comb has struck a knot (in hair) - and the implication is that now, either the comb will break - or you will lose a significant tuft of hair.

In other words a situation has reached a crisis or decisive or "tipping" point - and a decision has be made one way or the other.

There is no resiling from the situation at hand - it has to be addressed / dealt with.

"Matters have come to a head".