We need more information. What exact version of Lightroom are you using? What operating system? Have you opened the catalogue previously? Are you getting an error message? What is the exact wording of that error message?
If the old catalog was created by an older version of Lightroom then the new version of Lightroom will recognize that the catalog needs to be upgraded and will prompt you accordingly. Allow the upgrade to take place. That process will create a new copy of the catalog compatible with the new version of Lightroom. That new catalog will contain all the adjustments made previously, and will allow you to continue working with those images with the new version.