Praise be to Allah.
The general principle with regard to material damage is that the one who damages something is liable for it, whether he did it deliberately or by mistake, but there is no sin on one who did it by mistake.
A hired worker -- such as the plumber -- is not liable unless he overstepped the mark or was negligent. What you mentioned about him forgetting to close the main water line is regarded as negligence and carelessness, so he is liable for whatever was damaged as a result of that.
Al-‘Izz ibn ‘Abd al-Salaam (may Allah have mercy on him) said: Liability dictates compensation, and compensation is not waived in cases of forgetfulness. End quote.
Qawaa‘id al-Ahkaam, 2/4
Al-Zarkashi (may Allah have mercy on him) said: Forgetfulness means that there is no sin in cases of damage, but it does not mean that the person is not liable. End quote.
Al-Manthoor fi’l-Qawa‘id, 3/275.
In al-Mawsoo‘ah al-Fiqhiyyah (7/167) it says: If a hired worker is neglectful, then he is liable. End quote.
Based on that, the plumber is the one who is liable for the damage caused to the laptop. The basic principle with regard to liability is that what can be replaced must be replaced, and if the damaged item cannot be replaced, its value must be paid instead. So he has to buy something to replace the damaged part, if possible, otherwise he has to pay its value. The amount in question should be determined in consultation with experts.
And Allah knows best.