Housemarque wasn't kidding when it said it would have a patch for Returnal's save file bug within "a few hours." The studio has released Patch 1.3.4 for its PS5 title to revert to the safer 1.3.1 code while also fixing a keyboard exploit and a known Trophies problem. A proper 1.3.5 patch is coming soon, Housemarque said. However, whether or not your earlier saved games survived the flawed 1.3.3 patch is another matter.
If your saves maintained their file size after 1.3.3, you can resume progress even though they were likely corrupted. If they were reduced to 0KB, however, you'll have to resort to a PlayStation Plus cloud backup should you have one available. And if you don't... well, true to the game's loop-based mechanics, you'll have to start again.
The botched 1.3.3 update slightly tarnishes a launch for a game that's otherwise quite promising. Until now, the biggest frustration with Returnal was its brutal difficulty — and if all goes well, that will be your only consideration going forward.