Can my old employer just stop paying me the day the sale closes?

No. Up until the moment the sale actually closes, you are still employed by, and owed wages by, the seller, and the seller can't simply stop paying you because a deal is happening — you're entitled to be paid for all the time you actually work, right up to closing, regardless of the ownership change in progress.

What happens next depends on whether your employment is treated as continuing with the buyer. If the new owner is keeping you on as part of an operating business, Ontario's Employment Standards Act, 2000 treats your employment as continuous rather than as a termination followed by a fresh hire, so there shouldn't be a real gap in pay at all, just a change in who's issuing your paycheque. If your employment with the seller is actually ending and you are not being kept on, the seller still owes you everything earned up to your last day, plus whatever notice or other entitlements apply to an actual termination.

Either way, "the sale is closing" is never itself a legal excuse for wages simply stopping without explanation. If a payment is missed or delayed around closing, raise it immediately in writing and don't let it sit unaddressed.