Stuck in the startup state with the error message:
Started Hold until boot process finishes up
The reason is that the system disk is full.
The solution is to press Shift or ESC during startup, select Recovery Mode, and then select boot to enter the terminal. Use the command du -sh * to check which file is too large. Delete some files to free up space and the system will start up normally.
The reason for this strange problem is that on Saturday, I tested a training code through Pycharm, but it failed. I didn't have time to look at the reason and was called away. Today, when I checked, I found that ~/.PyCharm2019.2/system/python_stubs had 300+G. I deleted this directory and the system started up normally.