Firstly are all your upstairs and downstairs on the same circuit (using the same fuse) If so it sounds like you have lost power at the start of this circuit. Process of elimination you need to find out which light is first in circuit. I'd guess the closest to the fuse box, So you need to inspect these lights at the ceiling for loose or damaged connection. This is the first and most common thing to do. If still no joy, could be fault at a junction box if not loop in circuit.
This needs a little more experience to find where juntion box is situated.
Best of Luck
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