When you reboot your computer after completing your Mandrakelinux installation, you will first see a menu containing three or more items called the bootloader menu. This allows you to boot your GNU/Linux system, or any other operating systems you may have already installed, as well as some special options.
The number of items and their names will vary depending on your particular configuration. The one we are interested in at the moment is obviously the one labeled linux, which will start your Mandrakelinux system. It is the default item unless you manually configure it differently. All you need to do is to wait a few seconds — you will see a countdown at the bottom of the screen — or press Enter, and Mandrakelinux will start loading. You can select a different item by using the arrow keys on your keyboard and pressing the Enter key.