Engineers will work on plans this week (Sept. 5) to address the issues. Right now, Artemis is still on the launch pad. They will decide whether to work with it on the pad or whether to roll back to the VAB. Possible launch opportunities are listed on this NASA webpage.
There was a very large leak of hydrogen that looked like it was being caused by the nozzle not sitting properly in the connector. They tried a few different things to re-seat the nozzle but weren't able to stop the leak. Since they had only managed to fill hydrogen to 11% capacity, time had run out to fix the leak, fill to 100%, and launch within the 2:17 to 4:17 pm window.
The engines need to be cooled to -425 degrees Fahrenheit before launch. Three engines reported the correct temperature, but Engine #3 sensor showed it was not reaching temperature.
NASA determined that the problem is a temperature sensor. They decided that they will rely on the sensors in the other engines to ensure that they reach the correct temperature. They will also start fueling earlier to allow extra time for the engines to properly cool.
The 45th Weather Squadron provides the forecasts for launches. Here is their forecast for Artemis I.