I had set GIMP to single window mode a while ago. I opened an image file in GIMP recently and found that the toolbox, which usually appears on the left, was not displayed.
No one seems to know what causes this error. But, one of the solutions worked for me. Go to Edit > Preferences > Window Management and choose Reset saved window positions to default values. Close and open back GIMP and the toolbox should be back!
Tried with: GIMP 2.8.10 and Ubuntu 14.04