It may have accidentally been shifted/dragged off-screen or it’s on a monitor that’s now disconnected. AFAIK Roblox doesn’t have a way to reset the UI within Studio, but you should be able to reset it through the registry. Search for regedit in the start menu, open it, and navigate to
Computer\HKEY_CURRENT_USER\Software\Roblox\
The command bar geometry is stored somewhere inside RobloxStudio, but it doesn’t seem to be aptly named. You can either dig around for it, or delete the RobloxStudio folder entirely. This will reset all of your Studio settings to default – not just the widget geometry.