When can we reinstate our expired patent? How?

Check the patent section of the U.S. Patent and Trademark Office website. Since a patent is published at the time of registration and you allowed the registration to lapse, there may not be much you can do.

You can't, unless it was expired because of unintentional failure to pay a maintenance fee.

If the expiration was UNINTENTIONAL - then you have two years from the expiration date to seek revival of the patent. UNINTENTION means just that - you did not intend to have the patent expire. Maybe there was a record keeping mistake. Maybe someone who did not understand the situation ignored the reminder from the Patent Office. Maybe you never received the reminder because you moved. Check out the revival form at www.uspto.gov. Act quickly. GOOD LUCK!