What can cause a successful deployment to fail when redeploying?

Two days ago it deployed successfully, now getting ‘The CloudFormation template is invalid’
nothing changed in the config file serverless.yml

what part failed? was there more info?

I am getting:
Error: The CloudFormation template is invalid: Template format error: Unresolved resource dependencies [ApiGatewayResourceAppAnyVar] in the Resources block of the template