dddd

If your shifter is refusing to shift in either direction, first secure the whole thing to the end of your handlebar. It's hard to actuate one lever at a time with the lever body floating around, and if one lever is pushed part way by accident, the other won't work.

I can't imagine any damage occurring unless one of the levers was really forced. I'm assuming also that the shifter hasn't been sitting and is now refusing to shift because it's freezing cold (a mode of failure that often shows up one year as it gets colder. A cold shifter will work if the levers are pushed veeeery slowly, btw.

Many times a shifter will downshift fully (big brake lever) but will only upshift (small lever) to within one or two positions short of high gear. In this case, you could repeat the downshift-upshift routine a few times, and if it still doesn't make it to high gear (where the cable port lines up) you can turn the spool with the tip of a tiny screwdriver whilst upshifting with the small lever. Once the cable is installed, the shifter reaches high gear easily because the cable helps the spool along to high gear where the spool's spring starts to be insufficient to turn the spool by itself.
I usually only have to poke a spoke into the cable port and push on the cable head seat (simulating cable tension) while upshifting the small lever.