Helping people with computers... one answer at a time.
If Windows Task Manager doesn't have a menu or tabs, then it is probably running in what's called "Tiny Footprint" mode.
Why does Task Manager suddenly have no menu or tabs?
Windows Task Manager has a mode that maximizes the data display by hiding the menu and title bar.
It's easy to accidentally get Task Manager into that mode.
It's just as easy to get it out.
This is what you normally see when you open Task Manager. It varies slightly from version to version of Windows, but the issue that we're dealing with here has not.
Here's the version that you're probably seeing:
Note that there is no menu bar or window title bar. This is the so-called "tiny footprint"† mode.
Double-click anywhere in the empty area around the information display:
This is actually a toggle - double-click in normal mode and you'll get tiny footprint. Double-click in tiny footprint and you'll get normal.
Typically, that's the accident that got you here - somehow, the window was double-clicked on.
There's actually a Knowledgebase Article on it, based on Windows XP.
† No, I don't know why it's called "tiny footprint". It's certainly not smaller; in fact, the displayed data takes up more, not less, of the display area. Perhaps the idea is that you can then resize the tiny footprint window to be significantly smaller and still display meaningful data.
(This is an update to an article originally published June 20, 2004.)