I've deleted a tool from ToolBox by accident. How I can get it back if possible?
The standard way is to select the tab in the toolbox and select "Choose Items", this opens a window that allows you a list of controls to add, or you can browse to find the control in it's assembly if it is not there.
You can also restore the toolbox to the original configuration with a Right Click on the Toolbox and select "Reset Toolbox" (WARNING: This will give you the VS defaults. If you loaded any control libraries, you will lose them and have to reload them with their installer or "Choose Items").