My toshiba nb240 starts with a blank screen with power and hdd and wifi lights onn, but nothing can bee seen on the screen. and when i off it it goes off with a sound.

Hold down the Fn key (between the left Ctrl and Alt keys), and press F5 twice, (the first press brings up the present setting, the second selects the next option). This scrolls through the screen display selections LCD - CRT - LCD &CRT - TV - TV&LCD.
If this is set on TV, you will get no display on LCD (Screen) or CRT (external monitor)
Also, When using an external monitor, you may need to plug in the monitor, before cold booting your computer (not from hibernate or standby) so that your computer will "see" the monitor, and allow it to be used.

If your laptop appears to boot up and the hard disk LEDflashes, then the problem is probably a back light fault. This can be a faultyinverter or a faulty CCFL lamp. Please click on this link and follow myinstructions to identify the fault and possible fix :- http://www.fixya.com/support/r3598095-faulty_laptop_lcd_screen