Mailbox incomplete on opening

[#1]

My mailbox used to display all messages received in a  one page display.
Now it's only displaying some (? about 20) per page and unless
I notice there are more today, seven (shown in another page) I can miss reading
some of the received messages altogether.
How can I revert to seeing ALL messages in one page?.
How can I revert to NEWEST being shown at top of page?
Grateful for a step by step reply please !  Thank you.

[#2]

I assume you are using webmail.  To change the sort order of the list just click on the date column heading. If you find the earliest is now at the top click again to reverse the order.
That will probably fix the missing messages issue too

[#3]

You can elect to show more rows per page too by altering the setting under Personal Settings > Preferences > User Interface > Rows per page.