Outlook mail has stopped working on my iMac and requests my password

No need - just follow https://answers.microsoft.com/en-us/windows/forum/all/app-specific-password/fbb95691-f3ce-4546-967a-2780d0e59476 and create an App-specific password (for use on non-Microsoft desktop mail clients only). Or use the Microsoft Outlook app.

Welcome!

To effectively help in this setting where we can neither see nor touch your computer, we need more details.

What version of Outlook? Microsoft have taken down the activation servers for older version of Office products. Even though you have an activation key, you cannot access the server for older versions.

iMacs have been in continuous production for 26 years. In order to avoid non-productive guessing on our part, we need to know the sub-model and macOS version on the computer that is not responding. "About this Mac.. will show both. Example from macOS 12:

Obviously yours will say iMac rather than Macbook Pro. If the year in sub-model has modifiers like Early, Mid or Late, please include that.

⚠️ For security reasons, please DO NOT post a serial number.

Are the other Macs for which Outlook still works older? If so, what macOS version(s) are they running?

Also, if we cannot move your issue forward, remember that MS Office/Outlook is not an Apple product and there are not as many Outlook users active here as one would think.  MS Office questions posted in Apple’s forums can linger here a long time and still get no answer.

I believe you will be better and faster served by asking in Microsoft's Office forums:

https://answers.microsoft.com/en-us/msoffice

Be sure to include "mac" in your search string to limit the search to Outlook Mac topics. Similarly, if you post a question include "Mac" in the title. That makes it easier for other Mac users to search for new posts.

I believe you will find the contributors there knowledgeable and very helpful, much more so than MS's frustrating Windows-centric support documents.

Microsoft. This isn’t an ISP issue, has to do with OP not being setup with Microsoft’s App specific password.

Does their WebMail work?

https://courrielweb.videotron.com/cw/displayLoginEnLegacyResidentiel

Is this how you set itup?

1. Open Mail. Click on Mail then Add Account. 
2. Select Add Other Mail Account. Click Continue. 
3. In the field marked Full Name, type your name as you would like it to appear in your emails. 
4. In the field marked Email Address, enter your Videotron email address in lowercase and without accents. 
5. In the field marked Password, enter your password. 
6. Click Create. 
7. Click Done. Configuration will redirect to Mail homepage. 
8. Configuration is done. 

See https://answers.microsoft.com/en-us/windows/forum/all/app-specific-password/fbb95691-f3ce-4546-967a-2780d0e59476 on how to create an App-specific password for you to use with Apple Mail.

Who is your ISP?

So then, who provides your eMail ?