More than likely the Timeline is gone since Project player is now playing the Playlist (that is normal behavior when the timeline is not used). You can upload your project file here to verify that, but maybe you have a backup if you are taking advantage of Windows’ file history feature.
Unfortunately, we do not know what may have happened to it.
It is a good practice to save a backup of your progress periodically as you work. For example, every hour you could “Save As” you project and give it a new name - consider putting a date/time in the file name to remind yourself where that “snapshot” was taken.