You dragged it too large? Rebooting usually makes it maximum.
Or instead of rebooting use a Taskbar Tile command.
Minimize all windows but the one you want to maximize.
Also you don't specify if you like a "maximized" Restore size.
If you prefer a more usable size use the window's System menu's
Size command (Alt-Space,S,...) or the Taskbar's Cascade command.
Again, minimize all windows you don't want involved with the resizing
before executing a Cascade.