|What you're referring to is called the Taskbar. Do you have items on your desktop that are also disappearing?|
If so, it likely means the Explorer.exe service isn't running, or is running improperly. Open up Task Manager (Ctrl+Shift+Esc), direct yourself to the Processes tab, find explorer.exe (not to be confused with iexplore.exe) and press the End Process button.
After the process ends, go to the Applications tab, press New Task button, and start up the explorer.exe process again.
If you just mean the Taskbar is disappearing, you may find this guide helpful in understanding and modifying its settings. Read in particular the "Moving and Sizing the Taskbar" section.
Apologies if I don't respond to your reply immediately. I don't check this site daily, but you're welcome to PM me as a reminder.