Tracking someone down is generally not against the law- there are even professionals who do this for a fee. People often want to track down old friends, track people who owe them a debt, or whatever.
However, if it's a situation where stalking or harassment enter the equation, then it isn't legal and trying to contact them can be used against you. Generally, simply tracking someone down is not enough to prove this unless something else is going on or there is an order not to contact them, or some other circumstances apply.
So for instance, if you were trying to get back in touch with an old school friend who'd moved away and you'd sort of lost touch, and you ran into a relative of his, no drama. If you were trying to locate an ex who'd taken out a restraining order against you after you put her in hospital, then it's a completely different story.
Also, if someone is in witness protection or something to that effect, trying to track them down to intimidate them or something like that will land you in a world of trouble.