Hey @Lucy_Hui,
This error usually comes up when the server is under considerable load. You can simply restart your kernel a few times and it should work just fine. For further reference, check this thread out.
Just a future suggestion. You can always try to find the issue that you are facing on Discourse, since, it is highly possible that someone else might have asked the same question and it has been resolved. It will save you a great deal of time.
Regards,
Elemento