If a power line goes down or something happens where theres a loss of electrons in the power grid, how are the electrons replaced, where do they come from? Or is there a loss of electrons in the flow?
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There's no loss of electrons. Whenever some electrons flow out of one end of the wire, some more electrons enter through the other end of the wire. If this flow should stop at either end (or somewhere in the middle), it stops everywhere. You can't have a large excess of electrons in the wire, because their strong mutual repulsion prevents it; and you can't have a larg deficit of electrons either, because their strong attraction to the positively charged metal nuclei prevents it. It's not the same as when you cut a water hose in two, and the water dribbles out. The electrons can't dribble out.