My ubuntu does not start at once.I have to turn ON my laptop only to restart it so as to start my session.I do not know where the problem lies.

Could you explain in more detail what you mean? What happens when you initially turn your laptop on?

a blank purple screen appears(default of ubuntu) and nothing else...i have to restart by pressing the power button or else the laptop does not boot...

This is just not describing your problem at all.

How did you shut the thing down? Did you shut it down or suspend or whatever?

What is your hardware? Preferably not the make and model. There is no reason to expect folks to be familiar with your box or have to look it up.

Something had to preceed the purple screen if you shut the box down. What is it?

Do you see the menu screen? Have you tried booting to recovery mode?

nothing at all appears while i shut it down and even nothing when i start...no menu nothing...
it shuts down properly and even functions perfectly after the start...

You still haven't answered Widget's question.

Well, I think I will become a dentist. This really is like pulling teeth.

From this last post, added to your first post, one could assume easily that the problem is that you have to turn your laptop on to start your laptop. This is normal.

If this is not the actual problem take a look in you /home/<user name> directory in the "hidden" files. They will be visible if you hit Ctrl+h. Open .xsession-errors and .xsession-errors.old (all "hidden" files and directories start with a . ).

If they do not tell you anything post both of them here.

An answer to at least some of the questions I asked before would be nice. I did forget one and that is what version of Ubuntu are you using? All these things make a difference.

If you shut down again. When you boot up. You will get a bios screen. This is the one with the manufacturers logo on it. You should have some idea how long that stays up. Just before it goes to black hold down your "shift" key. Have another finger ready at the arrow keys. Your menu should appear. Instantly hit the down arrow.

This will stop the timer that will auto boot your system to the top menu entry. You want the second one if you can get to the menu. This is the recovery mode. See what happens.

SnapOn multigrips are brilliant for that

I will remembet that. Being a Blacksmith a hammer and punch have quite a bit of appeal. Tong like tools, though, have to be given a lot of respect.