If your system is installed properly and nothing relevant is deleted, then you can launch the desktop environment with the "startx" command.
Within the desktop environment you should find the proper administration tool for Mandriva to set the default login, which is currently a terminal login. You will need the graphical login option.
On some systems you can also set the graphical login to automatically login into a predefined user account (the old Mandrake systems had this, so Mandriva might have it too) - on single user PC's a reasonable feature.
I'm not using Mandriva currently, so I can't give more specific info.