I live in Ontario Canada; I was out of the country for about 3 years I just came back,

I have unpaid credit for the visa about $16,000 this is an old credit for about 3 years
I noticed the collection agencies tried to reach me during this time

What shall I do now?
What the collection agencies can do after this time?
What about Statute of Limitations (SOL) for Ontario on this credit?

Thanks,