GPT-4 has been completely non-responsive for myself and other ChatGPTPlus users, since at least yesterday morning pacific. GPT-3.5 continues to work normally. There have been a bunch of threads on the outage in the Reddit ChatGPT community (for example: https://www.reddit.com/r/ChatGPT/comments/12at7s8/chatgpt4_down/).
There has been no acknowledgment of the outage on the OpenAI status dashboard (https://status.openai.com/), and their Helpdesk is not accepting messages from ChatGPT-Plus users at this time (or at least not for me).