The corona problem arose now only whereas the lease agreement was entered three years ago and you were very well aware of the expiration of the lease period.
Thus it becomes your duty to be ready to return the deposit amount on the date it became due.
You cannot quote that there was no tenant coming forward hence you cannot return the amount.
That is your problem and not the problem of the tenant, hence you are liable to return the amount on the due date whether you got a new tenant or not.
You have no excuse nor there is any solution for this in law.
However you may request the tenant to bear with you to return the amount in due course of time sooner the national crisis comes to an end.
You may remember that you cannot stop your tenant from adopting legal steps to recover the mount with interest for the period of delay.