As Findecanor touched on, you need to clean up all of that flux residue and find where the trace connects to the pad you lifted, then carefully scratch the solder mask off of the trace until you have a nice metal surface to solder to. If you've never done it before, a small jumper wire would be easiest, but I have had good success even with excess cut off from the legs of leds, diodes, resistors, etc.
I don't do the whole modular LEGO keyboard build thing, but my guess is that maybe the trace runs to the pad, then continues beyond it to the pad for other space bar switch position, so lifting the one pad may have broken the connection to the other as well.