Usually those reservations are not refundable or exchangeable. In your case however, I think it should be possible, since the cancellation is probably the fault of Deutsche Bahn

You try to get your reclaim here, and make sure you have proof of the cancelled train:

https://www.bahn.com/en/booking-information/passenger-rights/passengers-rights-claim-form

If your train is cancelled, then you're entitled to a full refund of your seat reservation from the place where you bought it. If that was via Eurail, then you can do that here:

https://eurail.zendesk.com/hc/en-001/requests/new

If you bought it from DB, then use @BrendanDB’s link.

You're probably also entitled to compensation for delay, if you have a delay of at least 1 hour. You can claim that here:

https://www.eurail.com/en/help/delay-compensation