I cannot access my iCloud email messages before a certain date

I can only acess my received email messages after April of this year - the rest won't load whether on my Mail desktop or via logging into iCloud on the internet.

Does anyone know why this is and how to access my previous messages - I could access messages going back that much further, I have not changed my settins nor deleted the messages.

PLEASE can some help - I desperately need to get hold of some of the email which I cannot access.

Thankyou!