If you've never run into this, it can be a bit odd to describe, but sometimes you'll click or double-click to open a program and it will show up for a split second, and then seem to disappear except for a button or listing in your task bar. Most commonly, the program window is actually open, but is off the side of your monitor's display, out of view.

You can work with the program in full screen (maximized mode), but if you want to resize the program window, bring the program back onto your monitor by following these steps:

For Windows XP and Windows Vista:
1. Right-click on the task bar item for the program.
2. Select RESTORE. (If the Restore item is grayed-out, select Maximize, then right-click the task bar item to activate the Restore item.) 
3. Right-click on the task bar item again. 
4. Select MOVE. 
5. Press the left arrow key on your keyboard once. 
6. Move the mouse left across the screen (without pressing any mouse buttons). The program window should appear on your screen. 
7. Click the mouse once to position the window on the screen.

For Windows 7:
1. Click the listing for the program on the task bar to make sure it's selected.
2. When the thumbnail image appears, put the mouse pointer on the tumbnail, and right-click.
3. From the menu that comes up, select RESTORE. If the Restore item is grayed-out, select Maximize, then repeat steps 1 and 2, selecting  RESTORE on step 3 this time.
4. Once you've been able to select RESTORE, then Repeat steps 1 and 2, to get to menu of options for the program window, but this time, select MOVE
5. Press the left arrow key on your keyboard once. 
6. Move the mouse left across the screen (without pressing any mouse buttons). The program window should appear on your screen. 
7. Click the mouse once to position the window on the screen.

Again, this can be a tricky thing to spot, since a lot of times it will look like the program is either not actually open, or that it is perhaps hidden somehow. Trying these few steps first could potentially save you from spending a lot of time trying much more complicated solutions.