This is an impossible one to advise on really... you probably know that.
Normally screens crack for a reason... tight hinges and being twisted when opened etc, or pressure or weight on the lid (screen) having been applied at some point... which you say hasn't happened. Or the lid closed on some object left on the keypad etc.
I can only think of two courses of action...
If you are certain nothing has happened to the laptop then be prepared to seek an independent inspection with a company that would put in writing their findings... which you will have to pay for. That may reveal if there was some problem such as an incorrectly fitted screen (under mechanical stress etc). And before you do that notify the company whom your contract of sale is with... that's Ideal World, not Toshiba, telling them what you are doing and that based on the results of the inspection you are prepared to take proceedings against them. If you are 100% certain that the screen failed due to a manufacturing defect you have to tough it out... and prove it.
And secondly, before you do anything perhaps seek professional advise from somewhere first. Although you have rights you may be asked to prove the fault was present from new... which it wasn't.
Ultimately it's very unusual for a screen to just crack... that's not to say it can't happen... but I guess it would be the one in a million that had. Which it might have done :)
Good luck and let us know how you get on.
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