Outermost (containing) frames disappearing at will

Yesterday, I was designing a footer for the mobile version of my website (with the PC version already designed). Upon zooming out when done, I realised all outermost frames containing the rest of the design screens have suddenly disappeared.
Tried to undo but all undoing couldn’t reverse that, and my last saved version didn’t have the new screens I designed.
Is there a way to undo that without having to go to the last saved version and redo the work afresh?

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