I've also looked for an archive folder but can’t find that anywhere.
Sign into webmail and confirm folder subscriptions by:
select ☰ > Settings > ▸ Email
scroll down to foot of page
select Change IMAP subscriptions
expand all folder by selecting ▸ and confirm all folder are ✓ (ticked)
select Email (located top left)
review content of any newly subscribed to folders for missing emails
If the emails remain missing then unfortunately they are considered unrecoverable. Should you wish to investigate possible causes then look to see what commonality exists between you and others who have experienced the same issue.
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Thank you for your help - mine were all ticked so to no avail.
Oddly, although the emails have gone missing in the webmail and my ios devices, on the ios devices, if I can not see them but type in subject or key word they come back so I can open them.
For example, earlier I typed in "blinds" to find an email for an order I placed last year and the email which had been left in the inbox appeared and I could open it etc. When I closed it it was gone again (but I could get it back via search). If I try that on webmail it makes no difference.
In iOS Mail, is Filter by: Unreadon? select filter option as indicated by highlight (1) above to turn it off.
Apart from suggesting a check of Settings > Mail > Account > account name to confirm whether it is listed as IMAP or POP I am stumped as to the cause; if the latter is shown (POP) then emails were downloaded and stored local on your device.