I recently installed Ubuntu 7.04 on my system and for last few days been learning ways around this system and get familiar with it. I have to say I'm glad that my university has 25% machines running unix systems for Computer Science students or I would be lost.
Today I was messing little around and somehow changed GRUB. Since then GRUB menu just isn’t showing up on boot up, but system still boots and after 10 sec starts default OS or if I use cursors it will execute selected option.
What I did to get there? On the previous boot up I moved cursor up and down so time out been canceled. Then trying to see how the command prompt option work I pressed "c". I did not expect that I will have to call all the commands to start system:@ . Finding my self in this position I called one of few commands which I remmember "reboot". Since then I do not see GRUB:'( .
Any suggestions?

Found solution in this tutorial about GRUB

If you are also 'hiding' the grub menu, you will have 3,2, or 1 second to press 'Esc' to show the menu.