Between 4 and 5 years ago I found a story in which in short order, Harry failed to turn up at the sorting during first year, following this several teachers (including Minerva, possibly) went to check on Harry, but found that years earlier he'd died as a result of the actions of the Dursleys, and Vernon and Petunia were in jail but soon would be getting released, so they visited the unmarked grave of Harry, and buried him with his parents, before they discovered that there was still a chance to prevent the tragedy through traveling back in time about 10 years, and so Snape volunteered.
I think it was “Fixing Past Mistakes.” By DebsTheSlytherinSnapefan
“Harry didn't appear at Hogwarts causing concern. Albus immediately heads out to Privet Drive to find out what was going on, along with Minerva and a reluctant Severus Snape. What they find out changes everything for everyone in the wizarding world. Is there a chance for anyone to go back and fix past mistakes? is there any hope at all for the magical world?”
The scene with the Sorting Ceremony;
"Harry Potter!" Minerva McGonagall shouted, as she read his name from the register, one of the names she'd been looking forward to calling for. Finally the next Potter heir was at Hogwarts, and soon he would be in Gryffindor like his father and grandfather before him. Concern began to churn in her gut as nobody came forward; her blue eyes surveyed the room, looking for any sign of a child who resembled his father. Was this a prank? She looked at the list and mentally calculated the names before trying to count the wave of black-robed students still standing waiting to be sorted.
Albus Dumbledore sat up straighter, his own eyes blazing with concern as the child didn't step forward. He was unprepared for this; had his aunt and uncle not gotten him to the station in time? Or had they stupidly thought nobody would come calling? He would have to investigate further; no doubt Harry Potter would be at Hogwarts before night's end. He relaxed back into his seat, sure in the knowledge Harry would be fine. "Continue, Minerva; I'm sure Mr. Potter has simply missed the train," he said loud enough for the students to hear, his lips twitching when he noticed they all calmed down. It was so easy to calm them down; pulling the wool over their eyes was much easier than people in the Ministry, but as always he succeeded.