I’ve been pulling my (scarce) hair when suddenly the Developer Tools in IE8 didn’t appear as expected, on one day. The taskbar in Windows showed the tools’ window but there were none to be seen.
Luckily, a quick Google search unshoveled this StackOverflow question. The reason was that the tools were off-screen.
It was just a matter of right-clicking the taskbar tools’ window, choosing “Maximize” and pin the tools back to the main IE8 screen.