Do you ever plug a second monitor into that computer? I sometimes have my laptop with a second monitor plugged in while using Workbench. Then later, when I don't have the second monitor plugged in, I start Workbench and the icon shows in the taskbar, but the window is not on the laptop screen. Unfortunately, the computer has saved the monitor Workbench last used to put it on the same monitor the next time even if that monitor is not plugged in!
Is it possible that has happened to you?
To recover from this, I can plug the second monitor back in and find the Workbench window.
There is another trick I can use if I can't plug the second monitor back in. Right click on the icon and start a second copy of Workbench. Right click on the icon and select Maximize. That will snap a Workbench to the current monitor.