I am using visual studio 2017, To get the SSIS toolbox back, right-click on the SSIS design surface in the project and select SSIS Toolbox please see this link. This resolved it for me.
Adding the content below again, in case the webpage is not available in future.
I have installed the Data-Tier Application Framework and SQL Server
Data Tools for VS 2017 . I created a solution then returned to it a
week later and the SSIS toolbox had disappeared.I opened the toolbox window using View -> Toolbox but it was empty.
To get the SSIS toolbox back, right-click on the SSIS design surface
in the project and select SSIS Toolbox – hey presto! it reappears.