Synthetyser

Is it possible you had more than one monitor connected before, and the Synthesizer is off on a screen that's currently disconnected? Or perhaps it's not quite entirely off-screen—look around the edges to see if there's a corner still visible.

If you can't find it, the simplest way to fix things is probably to just revert to factory settings: https://musescore.org/en/handbook/revert-factory-settings