If you are using the sidekick on an author instance of CQ, you may notice that the toolbar at the bottom which includes the design and preview mode buttons, is not visible or available.
The functionality in the sidekick is determined by the ACLs (Access-Control-Lists) defined in the CQ server. If you do not have the appropriate privileges then functionality may be hidden in your sidekick. Sometimes the sidekick may become unstable if you add faulty components to the page, or after you have installed some packages that may have overwritten required objects in your application, breaking some dependencies.
You should first try to clear your browser cache, and then reload the page from WCM console to refresh the sidekick.
You should also ensure you have the correct privileges to access the appropriate design in /etc/designs. This can be changed by an administrator on the Users tab in the siteadmin console. If the privileges appear to be correct, then try to disable them, save, and then re-enable them and save. The sidekick should now display the toolbar again as expected.