An outpost on a 2G (gravity) water planet (95% of the world's surface covered by oceans) becomes stranded / abandoned by the Galaxy Government, probably forever. The warp gate in orbit broke, nobody is coming to help them, period. There's no supply chain issues because the issue is that's there's no supply chain.
99% of their power supply (per capita use before was 9 MWH/y) was nuclear. Because of some weird cosmic hiccup, all radioactive isotopes outside the planet mantle were forcefully stabilized. No more alpha, beta, or gamma emissions. Or fusion/antimatter/whatever.
The people on the surface, about two million humans, is stranded. Chemical rockets cannot reach orbit as the fuel mass required is more than the lift it could generate. And the ships they had all ran on nuclear fuel, which is as useless as a slag brick now.
They now need to contend with the planet's gravity as most of their power expenditure was to keep their living quarters in comfortable 1G. They have only 180GWH/year of installed power capacity available, (7,5kWH/month*person). Most of their databases and costly machinery are forcefully put offline.
The databases don't contain blueprints for heavy machinery, space vehicles, or sophisticated microprocessors. They have CNC, 3D printers, and tools, as well as the machines left on the planet. What they have in hands is usable but comparable to what someone back on mid XXI century had.
The atmosphere is breathable, the water is drinkable, and there's plants and animals imported from Terra. The colony and useless is in a comfortable tropical location, they don't have to worry about bad weather.
How long would it take for this colony to revert to a medieval society (technology (mostly) and/or customs)? Frame challenge: prove they don't.