Half of map despawning for seemingly no reason?

Hey! So, half my map random just stops existing whenever I compile and run my map in TF2. Here's some screenshots.

I was wondering if this is a common bug or if hammer has randomly decided I don't get to have a blu side. If there's anything you need to know I'll be happy to provide.

That was it, thank you very much!

Maid said:

You've probably got cordon turned on by accident.
https://developer.valvesoftware.com/wiki/Hammer_Cordon_Usage

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