You just installed Kali Linux and when you enter your username and password, you get a black/grey screen. No matter how long you wait, nothing changes. Here is how to fix it.
First, you need to boot into Kali Linux in recovery mode. This way, you can run commands from the terminal.
An internet connection may be needed for this step. To set that up, ensure that the network manager is running and then connect to your WiFi. The commands are below.
sudo service network-manager start
nmcli device wifi connect 'wifi_name_here' password 'wifi_password_here'
The last step is to install/reconfigure lightdm.
apt-get purge lightdm
apt-get install lightdm
During the installation process, a window will pop up and ask you to choose between gdm3 and lightdm. Choose lightdm for this and hard reboot when done.