It shows the “particle workbench: welcome” in the heading on the sidebar, but no window anymore.
hm. that's super-weird. couple of things you could check:
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from VSCode's main menu, select "Help" > "Toggle Developer Tools" then when that panel appears, switch over to the "console" tab, try to launch the "Welcome" screen, and look for any interesting errors
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from VSCode's main menu, select "View" > "Output", then try to launch the "Welcome" screen, and then cycle through the various logs (look for the drop-down menu in the panel toolbar top-right) and look for any interesting errors.
beyond that, i would simply recommend you confirm you are running the latest version of VSCode (as of this post, v1.40.1) and Particle Workbench (currently v1.7.0).
if you spot anything, please share it here and i'll take a look 