It is likely off your screen. Here is a macro that should reset it.
Code:
Sub StylesPaneDock()
' Charles Kenyon
With ActiveDocument.CommandBars("Styles")
.Visible = True
.Top = 0
.Left = 0
.Width = 260
.Position = 2
End With
End Sub
Instructions for Installing Macros from Forums or Websites by Graham Mayor, MVP
This happens with many panes and dialogs from time to time. In vba, they are called commandbars.