If your Eurostar train was canceled, your options are:
Whichever option you choose, your first step is to get in touch with us, through our contact page. Please include your 6-character booking reference. If you’d like to get your tickets exchanged, please also mention which new travel date and train time(s) you’d like. We’ll answer you as soon as possible.
In the case of an emergency, you can also contact Eurostar, via Twitter for instance, to ask for an exchange. They may not agree to the exchange, however, and tell you to get in touch with us. In any case, you should also be able to ask Eurostar staff at the station directly to exchange your ticket.