Perhaps you inadvertently clicked the link button on the clip thumbnail. It's on the right here... 
To undo mistakes, step-by-step, press <ctrl-Z> or EDIT | UNDO from the menu bar.
Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the chevron to the right of the Save Project button on the toolbar. Then you're protected, and can revert if need be.
About the only way that comes to mind to prevent accidental unlinking is to lock the track, but that also prevents editing.