White Loaf gone Wrong

400g very strong canadian flour

75g plain flour

25g strong wholemeal 

360g water

10g salt

45g starter 

I fed my starter and it doubled, however it didn't have much structure to it so perhaps that is where it first went wrong. I autolysed for 30 minutes then added the starter and salt (5.30pm). Then I let it rest for 2ish hours. Then a coil fold. I let it rest (until the dough had relaxed) and then coil folded around 2 more times. Then I let it rest overnight and the next morning (5.30am), I preshaped, bench rest, and then final shaped for the cold ferment for exactly 12 hours. Before the cold ferment, the dough held it's structure well and was light bouncy and glossy. 

When I took the dough out from the fridge the dough lost all it's shape and structure and completely like spread out. I could barely manage to move it into my cast iron. 

The final bread didn't have too much flavour and was quite chewy. It makes for a nice piece of toast I guess though!

Is this overfermentation, underfermentation? Where did I go wrong, did I push the cold ferment too long? Was it to do with my starter ?

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