VOCALOID3 Editor might already be running.

Press the Alt + Tab keys to switch between tasks and check if VOCALOID3 Editor is running.

Also, if you were using a multi-display environment previously, the Editor screen might not appear (might be off screen) even if you select VOCALOID3 Editor by switching tasks.

In that case, you might be able to find the Editor screen if you select VOCALOID3 Editor by switching tasks, and then press the Alt + space keys, select "Move" from the menu that appears, and use the cursor keys on the keyboard to move the Editor screen.