Looks like the save file got corrupted. vTuber kit does make backups every time you open the program, so your old save file might be okay. Follow the video below to find the backup folders. Sort by date because you wanna find one from before the incident occurred. If one doesn't exist, you're just gonna have to use the "erase all data and start over button" I think. But if you find one, copy that whole folder to the desktop to make sure it's safe. Then try copying over just the .uwu file to the vTuber kit folder and replace that save file and see if that fixes the issue. If not, try copying everything from that backup folder into the vtuber kit folder.