We were kicked out of Lassen Park, no warning. Just signs and threat of arrest if we crossed the entrance. Many visitors where removed from the hotel at Yosemite, and the parks were closed.
Most of the campgrounds were shut down, but a few remained open as long as possible down the Rte 395 spine on east side of Sierras because so many campers were without a place to camp while returning home, including us. Rest areas and other turn-outs filled each night with campers locked out of reserved sites.
State parks filled immediately, as did most private parks ... so it was tough even to find a back-up. We were in California at the time, on a coast to coast trip from Florida. We limped home catching open camp areas occasionally (no national camp areas) and over night rest stops. Not a pleasant experience.