It all depends upon the wording of the agreement you had with the first architect. If he was contracted to see the job through to completion, he should provide the final certificate. If he wasn’t, he doesn’t have any obligation to do so. But in any case, it seems unfair on you if he evades his responsibility by retiring (unless the cause was sudden ill health) without letting you have notice of his intentions. Perhaps a letter from a solicitor may produce a result for you.