If a clip is linked, deleting or repositioning a portion of either audio or video will affect both, and sync is preserved. If sync is out, they were not linked.
Sync can be restored by a right-click on the clip, then choosing "Restore linked..."
For protection, 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, the AS choice is available by clicking the caret to the right of the Save Project button on the toolbar. Then you can revert if things go bad.