If you purchased a Eurostar reservation for passholders and no longer hold an active Eurail pass (with activated travel day for that day), you simply do not have a valid ticket. You might be denied boarding and/or fined for ticketing fraud. You either buy a new pass (in that case it would have been better to request an exchange) or reach out to customer service to whoever you bought the reservation from or Eurostar themselves, and ask whether it can be changed for a regular ticket. Eurostar seat reservations are non-refundable, but can be exchanged once for a €15/€20 fee + the difference in fare - possibly this includes changing them to normal (non-passholder) tickets, but I’m not sure.

Note regular tickets on Eurostar are generally considerably more expensive than the €30-40 seat reservation fee.

You can check this on your Eurostar tickets. My passholder reservation clearly states: “Holder of valid Eurail/Interrail Pass only. Exchangeable before departure by paying a £15/20€ fee per leg per person, non refundable.”