Perhaps you’ve docked it by dragging it right until the mouse pointer is at the edge of the screen, drag it back to undock it. Also, right-click the Taskbar > Show windows side by side and Undo Show side by side does the same thing. Shaking the window also restores it to its normal position.
Here are a few key combinations:
Windows Key+Up Maximize vertically
Windows Key+Down Restore/Minimize
Windows Key+Left Dock to left
Windows Key+Right Dock to right
Windows Key+Home Minimize Restore all other windows