Did you mention to customer service when you want to travel? Since that is the most important factor to determine the urgency of a request.

In future, don't activate your pass or a travel day unless you’re sure you are going to use it. You can only cancel a travel day until midnight CET before it starts.

Hi FloraS, I hope you are feeling better now. Our customer service is experiencing serious delays, did someone get back to you in the meantime? Take care and cheers,

Hello Flora

Please can you let us know if your pass was activated before changing you mind to temporary postpone your travel ? 
If your pass hasn’t been activated on the first day of travel eg outbound journey then you can travel on a suitable for you date. However, your reservation for Eurostar if your planned to travel to France/ Belgium would need to be bought again if you skipped outbound travel day. 

This information is likely too late to be useful, but in case other travelers have the same question: There is no way to reset or pause a pass after it has been activated. You can cancel a planned travel day up until 23:59 before the first travel day starts. This will also ensure that the pass remains inactive. Of course, we are ready to help when the unexpected happens. You can reach out to customer service so that we can either help you exchange your pass for a new one, or add an extra travel day once we've verified that the day was unused. We hope you're feeling much better, and please do let us know if we can be of any help :)