What happens if a business partner dies and there was never a buy-sell agreement in Ontario?

Without a buy-sell agreement, there's no pre-agreed mechanism forcing anyone to buy or sell anything, so the deceased partner's shares or ownership interest simply becomes part of their estate and passes to whoever inherits under their will, or on intestacy. The surviving owners don't get an automatic right to buy those shares back, and the deceased's estate trustee, or eventually the beneficiaries, don't get an automatic right to force a buyout either; both sides are left to negotiate from scratch, often while grieving and under financial pressure.

That can mean the surviving owners end up in business with the deceased's estate or family members who have no interest in or ability to run the company, or it can mean a standoff where neither side can afford to buy the other out. Resolving it usually comes down to negotiation, a professional valuation to anchor any deal, and general corporate or partnership law remedies if talks break down, including, in some cases, an application to the court. It's a much slower and less predictable process than a properly drafted agreement would have provided, which is exactly why buy-sell agreements are recommended before this situation arises, not after.