well at first I was gonna tell you that you've likely just damaged the nerves in your forearm a bit by landing awkwardly on the bag.
but then I reread your post,HANDS as in plural? As in both your hands have been numb and tingly for days? It is uncommon to hurt both hands in the same exact fashion at the same time, but not impossible. it doesn't rule out traumatic injury as the underlying cause, but it does make it far less likely, therefore it is possible your symptoms are being caused by something other than boxing (IE diabetes, alcoholism, vitamin b12 deficiency etc).
Obviously some of these causes might not apply to you, for example if you dont drink, but ya you might want to get it checked out, especially if you think its gotten worse not better since the incident, or if you have a history of cardiovascular disease,diabetes, or obesity or anything like that. Again not to worry you, but it is unusual that you would physically injure both hands, and numbness/ tingling of the hands or feet can be associated with some bad diseases.