The bridge, between junctions three and four, came down on the London-bound carriageway on Saturday after it was hit by a lorry carrying a digger.
A motorcyclist was hurt but his injuries were not life-threatening.
Two large cranes worked to clear the road, which is the main route to the Channel Tunnel and Port of Dover.
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The crash and closure of the motorway happened on one of the busiest travel days of the year.
Julie Maddocks, from the Freight Transport Association (FTA), said the 24-hour closure and delays would have a big economic impact.
"We saw yesterday 15 miles of tailbacks and it costs a pound a minute to run a 44-tonne truck," she said.
"So if you look at that five hours stuck in traffic, that's around £300 - that's just one vehicle."
She added that tourists and lorry drivers also missed ferries, which meant both would have incurred extra costs.
Witness Alex Magaisa was passing the lorry when the bridge collapsed.
"My instinct was just to drive through. My wife saw the bridge falling and there was a big noise. I had to manoeuvre through the debris," he said.
"It was a big shock. It's only just starting to sink in now what might have happened. We could have been crushed."
Part of the bridge fell on to a second lorry, the driver of which was treated for shock at the scene.
A motorcyclist in his 50s was taken to hospital with suspected broken ribs.
Sonny Gladdish, who was just behind the crash when it happened, said the motorcyclist had thrown himself off his bike to avoid the impact.
No-one has been arrested in connection with the incident.
Kent Police said it was investigating and appealed for witnesses to get in touch by calling the appeal line on 01622 798538 quoting the reference 27-0690.
The M20 in Kent has fully reopened in both directions following a crash and collapse of a footbridge.