With the most Simplicity Studio release (January 26th, 2018) there have been several cases of Simplicity Studio hanging when it starts up with the progress indicator saying Simplicity Studio is refreshing the workspace. Silicon Labs has identified and fixed the issue which will be in the next Simplicity Studio release scheduled sometime around February 23rd.
If this lockup occurs, the following work around can be used until the release is available. After exiting Simplicity Studio and using Task Manager if necessary to kill the Progress Information window, use a new workspace by renaming (not deleting as the workspace contains all of your projects) the existing workspace. The default workspace for Windows is at C:\Users\[USERNAME]\SimplicityStudio\v4_workspace. Existing projects can then be imported from the renamed workspace using [File] > [Import...] > [General] > [Existing Projects into Workspace] and then clicking the "Select root directory" radio button and browsing to the location of the renamed workspace. The detected projects will be listed in the Projects window and you can select which projects to import. You should click the option "Copy projects into workspace" and then click Finish.
The lockup can happen (but doesn't always happen) if more than one project is open when Simplicity Studio starts up. So to prevent the issue from occurring again, close all but the current project before exiting Simplicity Studio. The easiest way to close the other projects is to right click on the existing project folder in the Project Explorer and select "Close Unrelated Projects".
Silicon Labs apologizes for the inconvenience this issue causes our customers. This KBA will be updated when the fixed release is published.