This question has been asked many times and saw this on the DC Embassy page, hope it helps...
DC Embassy:
Flight cancelled? Please contact us for a revised COE. Travelers who have received COEs but have to change plan due to flight cancellation should write to the embassy / consulates-General. They will be given special access to their old accounts on the COE application platform to amend flight/ASQ details and re-apply for a new COE.
TLDR : Answer Summary
If your flight to Thailand is canceled, you should contact the embassy or consulate-general to obtain a revised Certificate of Entry (COE). You can gain special access to amend flight and ASQ details on the COE application platform. Note that if your flight changes within 72 hours of departure, a new COE is not required. It’s also advisable to reach out directly to the embassy for assistance, especially if you’re currently in another country and facing entry issues.