this is an answer to an old question for completion sake. 7 years later and we still have questions similar to this.
The reason while you see the cracks when your tablet turns on , is because the crack affects only the LCD. Not the outer glass and not the digitzer which are adhered to the LCD. In cases like this a replacement of the complete display panel would be advisable. Since we do not know the exact model of this Coby tablet and since it is a bit off-brand it might be a bit more difficult to find a replacement.