Make sure the app is running by looking for the Upwork logo in the taskbar. (This is located in various places, depending on which operating system you're using.)
If you see this icon, you can double-click it on Windows to bring up the time tracking tool. On a Mac, I think you can click and choose to view the timer tracking tool from a menu that appears.
If you do not see the icon, be sure you are running the Upwork app and try again.