Many of those taking to X to discuss the outage were making light of the situation. One user called Mustafa asked: "ChatGPT is down again? During the work day? So you're telling me I have to… THINK?!"
The outage began around 11:00 GMT on Thursday, and users continue to report issues.
ChatGPT's status page said the app had been "experiencing elevated error rates" and the firm was "currently investigating".
Though many people use the free version of the chatbot, the firm charges up to $200 per month for its various membership tiers.
OpenAI boss Sam Altman said in December that ChatGPT is used weekly by more than 300 million people worldwide.
It comes after tech firms including OpenAI pledged a $500bn (£405bn) investment into AI infrastructure in the US.