Smoke from engine, cranks but wont start

I came off the motorway to a slip road, slowing down to 20mph and about to turn when I saw smoke coming from my engine bay. Then all the lights on my dashboard (not just warning but all of them) starts blinking and without pressing the brakes, my car starts shutting down and coming to a halt. Good thing this wasn’t on the motorway.

Popped the hood and smoke was coming from the left side (passenger side – am in the UK by the way). There was smell close to the alternator which I found out was melted tape that has been heated up. Left car to cool for 2hrs. Check the engine bay and nothing seemed out of the ordinary. Slide key in, turned on but not starting the car just yet. All seem good on the dash. Tried to start it but all it did was crank. Bought spark plugs and battery but it was the same result. Towed to mechanic, who said it was cat converter when I cranked it in his presence. Tow truck guy even sprayed something, cant remember the name but I think it was an oxygen spray into the air hose but still nothing

Before I start buying a new one and having him on it, just wanted to get a second opinion