First; the timeline is not locked, it simply contains no sequences.
If you importerd an old project its likely the project's sequences are in a folder. Locate the sequences in the project bin(s) and double click on them. That will add them to he timeline.
Welcome to the forums.
Curt has answered your question accurately and completely, but I wanted you to be aware that you will also find links to many free tutorials in the PremiereProPedia that will quickly show you how things are done in Premiere Pro.
Managing sequences in a project is a very basic skill, in PPro, that you need to get the knack of.
Though mentioned in both replies, imprint that "Dbl-click" of the Timeline in Project Panel into your mind. It has caused many "Oh shoot!" moments, when one thinks that all Timelines are MIA. Even trips up a few of the experienced users from time to time.