The App Store application dowloaded over the Microsoft Word I already had

Restore your Word app from your backup that was made just prior to when the download occurred. You do have backups, right?

If that doesn't work you may have to erase your HD, reinstall the system and migrate your files from the backup just prior to the occurrence. How to reinstall macOS from macOS Recovery - Apple Support

Uninstall Office 365 and reinstall the version of Office you had. Use the same license. Confirm on the Microsoft product help site. It is a Microsoft product and license. Everyone in those communities knows how to reinstall Office and reuse the license.

I have never heard of having to reinstall an OS in order to reinstall Office.

Tayanaa wrote:

That's the thing , I do not have office 365. The Microsoft apps came preinstalled on my laptop

In your original post you said Office was downloaded from the app store and installed over your original Office installation. If Office 365 was not installed then just use Office as you always have.

If there is more to the story than you've provided, such as you upgraded your OS to Catalina and now your old Office version 2011 no longer works then: Office 2016 (upgraded), Office 2019, and Office 365 will work with Catalina. You can buy Office from Microsoft.

If you have Time Machine enable and running you can restore your MacBook to the version it was before you took it into the tech guy (in the future I'd learn to do that stuff yourself or get a new tech guy).

You should be able to go to the MS Office site and with your activation code download and reinstall the version of MS Office you had.

Alternatives to Word are LibreOffice and Pages. Both can open, read, edit and save Word documents as Word documents or their own format. Both are free.

Since there are a few possibilities, it's a bit difficult to figure out what you did. Office 365, 2019 and 2016 all put the individual Office apps in the Applications folder. Worse, they all have the same names, so it's easy to overwrite one version with another.

Since you say you've never purchased Office 365, that can only mean you overwrote Word from Office 2016 or 2019. Assuming this is correct, download the appropriate software from Microsoft's site and reinstall the latest version.

Office 2016

Office 2019

Tayanaa wrote:

That's the thing , I do not have office 365. The Microsoft apps came preinstalled on my laptop

If those apps were "preinstalled" and were working then you are using a pirated license, unless your are using your employer's license. Who, exactly, installed those Office apps for you? You simply cannot use any of those apps fully without paying for a license or subscription from Microsoft.

You can find out which version of Word you have by setting your Finder to Column View and it will show in the right hand pane:

Note the Version heading at the bottom on the right.

Microsoft Office is not an Apple product. You might try asking in the Microsoft product Help Communities.

If your tech guy originally installed all of the Microsoft apps free of charge, then they weren't legal in the first place.