Is Zakat obligatory on a sum of money in a bank account that has reached the Nisab of Zakat and upon which a Hawl (full year) has passed, knowing that its owner cannot withdraw or spend from it except a limited monthly amount? And is Zakat obligatory if the money reaches the Nisab of silver but does not reach the Nisab of gold?

If the money in the account is genuinely owned but cannot be withdrawn, then Zakat is not due on it now. Rather, it should be paid for one year after it is received. This is what Sheikh Ibn Uthaymeen indicated, although there is another opinion that obligates Zakat for all past years. As for the amount that can be withdrawn, whatever was spent from it before the completion of the hawl (one lunar year) is not subject to Zakat. Whatever remains of it and upon which the hawl has passed is subject to Zakat. The nisab (minimum threshold) is estimated by the lesser of the two nisabs (currently silver), which is approximately equal to 85 grams of gold, or 595 grams of silver, and Zakat of one-quarter of one-tenth (2.5%) is due on it.