DIAGNOSIS The weatherproof seal around the camera lens/dome has failed. This usually happens over time with temperature cycles, or if the camera was mounted at an angle where water can pool against the seam.
FIX Unfortunately, there's no user-serviceable way to reseal the lens compartment. Your main path is a warranty claim. 1) Document the issue with a photo of the foggy lens and a screenshot of the blurry video feed. 2) Contact Google Nest support. If the camera is within its warranty period and shows signs of seal failure (not physical damage), they should replace the unit. Be prepared for them to ask for the serial number and proof of purchase.
IF THAT DIDN'T WORK If you're out of warranty, some owners have had limited success by carefully removing the camera (unplugging it first) and placing it in a sealed bag with silica gel packets (the kind that come in shoe boxes) for a day or two. This can draw the moisture out, but it won't fix the broken seal, so the fog will likely return.
HEADS UP Don't try to pry the dome open or use heat like a hair dryer. You'll almost certainly damage the lens coatings or warp the plastic, making it impossible to get a good seal even if you could reassemble it.