Hi, welcome to the community!
Many users (including me) encountered with that bug.
Using following method, the issue was fixed, project folders were visible again on sidebar. You may try.
If it is not fixed, you can contact with OpenAI support team:
To contact with OpenAI Help Center and start a chat using widget on bottom-right side of the page with OpenAI support, follow steps below:
1 - Go to ChatGPT and log in with the account you want to use.
2- Go to Help Center and select the Chat widget icon.
3- Click on Messages
4- Click on Ask a question
and continue…