I'll try to clarify.

Webmail is showing messages in its own Inbox, which MS Outlook is not showing in its Inbox. (Only a small number of messages -- or, I should say, only a small number of contacts' messages -- for each account is reaching Outlook).

F9 does nothing when I press it in Outlook. Send/Receive All downloads some, but not all incoming messages. Individual accounts seem to be affected. Most but not all incoming messages do not make it to Outlook. No error messages visible in Outlook or on webmail.

Ctrl+6 rearranged the columns in Outlook's Inbox a bit. I'm not sure what it did, but I did not see anything that looked like an error or error notice and I don't know if there is a specific place where I should be looking for them.

I think, at some point, I must have clicked a checkbox to stop processing scripts in one of my Rules -- but there are easily hundreds, if not thousands of them, to look through to find it. If this is the case, I would expect it to be in a recent rule, since I became aware of missing messages recently. But the problem is that I don't know what I don't know and my safest bet would be to delete all rules (which could also take a lot of time, if I have to delete them each individually). And if, after having done that, I am still losing messages, that suggests the problem could be somewhere else and I deleted all the rules for nothing.

I'm just trying to figure out the best, easiest, and safest way to figure out what caused the problem and how to resolve it before I do anything further.