Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello, Nadia Jussub
Welcome to Microsoft Community.
Microsoft 365 is a subscription service, not a one-time purchase. You can't buy it once and use it forever; the longest subscription period is five years. If you bought a version from 2014 or 2015, even if you find the key, it might already be expired.
If you need to keep using Microsoft 365, just resubscribe with your own account:
How to renew Microsoft 365 for personal or family use - Microsoft Support
Yuhao Li
Microsoft Community Technical Support