The message and attachment have separate outcomes
A message summary can show an attachment count without carrying the attachment files or their links. Opening the message provides more detail, but an attachment that failed storage has no URL to download. This does not mean the email was lost; it means the attachment could not be retained with it.
The documented storage causes are an attachment over 25 MB or a full mailbox quota. If you know the sender, ask them to share the file through an approved file service or resend a smaller version after space is available. Do not assume that retrying the message view can recreate bytes that were never stored.
Avoid exposing the file through an unsafe route
Before asking for a resend, check whether the mailbox has available capacity and whether the sender's attachment exceeds the size limit. For sensitive information, use a secure sharing method approved for your organization instead of forwarding a private file to an unverified address.
If the message is important for records, keep the message and note that the attachment was unavailable. A missing download link should not be treated as proof that the sender omitted the file or that the message was malicious; confirm with the sender when the attachment matters to the task.