Hello,
Thank you for coming to the forum.
First, I will advise you to do a reboot and try and open the old files from within the apps.
Click Start > type Word > Click the Word app then click File > Open > browse to the file. Give us a reply if the files open or not. If you do not see Word or Excel when you click start read the below.
Second, please use the Word app or Excel app, not Microsoft 365 (Office) app. That app [Microsoft 365 (Office)] is for Office Web, which is not the same thing as the Office Desktop app, the Desktop app is what you need to open files on your computer. Microsoft 365 (Office) app is for files saved in OneDrive.
If when you type Word in the start, you do not see it then you will need to install it, click the link below and follow the steps:
https://support.office.com/article/4414eaaf-0478-48be-9c42-23adc4716658
After the above installation and following the steps to activate the Office 365, try opening the old files again by double-clicking them
If you do have the apps installed, do as I said and try and open the files from within the app and give a reply
Best Regards
Fuad