You contact Oribitz here, BA can't touch the TA's tickets. Orbitz should offer (by law) a refund back to you if BA cancel flights into or out of Europe. So you can leave it until the lines get less busy, but you are entitled to a full cash refund. The voucher is an option too but I can't think of a good reason to have that.
More information in the main covid-19 rebooking thread.