With Debian I assume you mean the console based(command line) login screen.
This could have been due to various reasons:
1) Low disk space in the home directory and/or root directory
Solution: Login through the debian console and delete some large files using rm command. Restart the laptop.
2) Filesystem permissions altered
Solution: Login through the console and run this at the home directory "sudo chown -R $USER.$USER . "
Restart the laptop.
3) Issue with X server
Solution: Login through console and run 'startx' command, see if you get the KDE login screen