There could be several reasons why this is happening.
First, switch to a TTY and run a full sudo pacman -Syu
again to make sure the system is fully up-to-date. There is more detail and further options for this step under [FAQ] Computer doesn’t boot, boots to a black screen, or stops at a message
Then, sudo chown -R $USER $HOME
to make sure file and directory permissions are correct.
If neither of these steps work then you’ll need to find some log files to determine what is preventing login, but the steps above are a good starting point.