i recently closed down a restaurant after 20 years, a new tenant is now opening using the same name. is this legal?

Did you have the trade name registered at the Secretary of State's office?
Were you operating under the name of an entity such as an LLC or a Corporation?
Did you have an asset acquisition agreement with the new owner?
Did you have a d/b/a registered?
If the answer to all of these is no, then there's little you can do. If the answer to any of tehse is yes, then I advise you to retain a business attorney to pursue your rights to the name.

This answer is provided for informational purposes only. True legal advice can only be provided in an office consultation by an attorney licensed in your jurisdiction and with experience in the area of law in which your concern lies.

Trademarks in restaurant names are unfortunately notoriously difficult to protect. Without knowing the actual name at issue, it is somewhat difficult to advise you. However, the key to having rights in a business name (aka a trademark) is "use." This means that one has to continuously use the trademark for it to be protectable/enforceable from or against another party. Given the current hiatus, that may be in itself a hurdle. On the other hand, the key to asserting "infringement" is "likelihood of confusion", meaning that if consumers would be confused into thinking that the restaurant is run by the former management, you might have a case (but one that might not yield you any monetary damages, given your hiatus again). Hence, you should likely consult with an attorney to look over all the facts/details.

I hope that this helps.

Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.