What is the ruling of Islamic law on a an accountant not reporting the loss of a sum of the company's money, and is he liable to guarantee this amount?

The hand of the questioner over the company's funds is a hand of trust (amanah); he is not liable for it except in cases of transgression or negligence. If he has not been negligent or transgressive in safeguarding the amounts, then he is not religiously liable for them, based on the statement of Ibn al-Qass and others: "Every property that perishes while in the hand of a trustee, without transgression, incurs no liability upon him." He is not obligated to inform the company if he fears harm.