I am so sorry. My heart goes out to you and your staff, but especially to the man's loved ones. Death is always tragic, for there is always someone left that suffers loss. I know you are hurting now and like many have said, it is NOT your fault. As a Pastor, I know your guilt won't just end because we are telling you that it is not your fault - however, it is important to hear.
In the meantime, allow yourself to grieve, but please don't despair. As bad as this incident was, there was nothing that could have prevented it. If you are a Christian, you probably know already that nobody dies until his appointed time. When that time comes, there is nothing that can prevent it. Even if you aren't Christian, you may understand this. You couldn't stop it,,but you can change how it effects you going forward.
Go about your business. Continue to provide the best food and service possible, just like yesterday. Don't let this bring down what you have worked so hard for - a business that so many count on. Don't let it seal your joy, even as you temporarily mourn. Do not dwell on what happened, or "what-ifs," for you can change nothing. Work for the future. Keep moving forward.
Your pain will subside - in your own time. Until then, remember you are not alone. The. Brethren are always here for you - if you need to vent, rant, cry, or cuss at someone, let us know. Heck, I'll give you my phone number for any of those reasons, or if you just want to talk or pray. In the meantime, you, your family and employees, and his family will be on my prayers. God rest his soul.
The Lord be with you and keep you!