The USPS normally keeps the tracking records and tracking numbers in their system for a period of 1 year or a bit longer but usually no longer than 2 years.
After 1 year normally the USPS tracking records and tracking numbers are purged from their system to make room for more tracking information.
The tracking numbers eventually get recycled after a year and go back into their system for other people to use for their packages.
So it's a good chance that the tracking number you have now was used by someone else.