Depending on the model of the Toshiba...normally this is caused by a defective inverter and should be replaced by a qualified PC Tech.
UPDATE:
In case anyone is wondering...I've worked on Toshibas a lot...it seems the LCD inverter is what causes the problem the OP described...in most cases. However, this alone may not be the issue.
Again...a qualified Tech can replace the relatively inexpensive part (depending on where you obtain it). If this doesn't solve the issue...then you may need a new LCD...but let the Tech decide.
