If you want to travel, Eurostar is obliged to rebook you on the next available service without extra costs. Or on a future date of your choice. Please note that this rebooking does not depend on availability of pass holder seats: they're obliged to rebook you on a service of your choice whenever there are enough free seats available. I'd call them to do this. It may also be possible to rebook yourself via Eurostar's Manage your booking as they suggest on this page.

If you want a refund, contact Interrail's customer service. They can't make you pay for a service that does not exist anymore, so you're entitled to a full refund. If you do that though, then you'll have to do any new booking yourself and you're limited by pass holder seat availability.

As to travel days: it it not a good idea to activate them in advance because of events like this. If an activated travel day is still in the future, you can cancel it in My Pass: select it and tap on Cancel travel day. If your activated but unused travel day is for today (or in the past), then ask customer service to fix it.