stuck at SDDM

Before today, I was using Netrunner 14, the day before yesterday I opened my computer and found that I can not pass kdm login interface with correct user name and correct password, but I did be able to log in via tty ( Ctrl-Alt-F1), but after logging in via tty, I pushed Alt-F7 to attempt to return to X server graphic interface, but I found I was led back to kdm’s login interface . Then I gave up Netrunner 14 and fresh installed Netrunner 16, but after update and installing several application, restarted the computer, I was stuck at SDDM’s login interface again ! Even though I knew I was using the correct user name and password. I am shocked that the same issue could occur in both Neturnner 14 and 16 ! Help !

Does it give any error message ? Like wrong password or username or something ?
Are you really sure you are typing in the correct passwords and username ? Is the keyboard layout correctly setup even in login screen or is it maybe different there ?

No error message. I am sure I have typed in the correct credentials. Because I can not access to the computer now, I will check tomorrow to see if there is some issue related to keyboard layout.
In the meantime, I went to kubuntu forum today to see if I could find the solution, since the netrunner I installed is based on kubuntu. And I did found the same problem happened to kubuntu users, and the solution include removing the Nvidia proprietory driver, changing display manager,removing old .cache, old .kde or .config direcotry etc. Tomorrow I will try them one by one .
Nvidia closed-source driver is really annoying !

I finally solve this problem by adding nomodeset into grub2. Becasue I am using a very old Nvidia graphic card (Quadro FX 350M), in some cases ,if from the beginning kernel using KMS handles video output , then I will not able to log in normally. So VGA driver is not the cause of this issue, no matter proprietary driver or open source driver, without “nomodeset” presented in grub.conf file, this machine will not be able to log into system . It took me almost one day to figure this out ! it reminds me how old this laptop really is, its wireless card is bcm 4311, it can not work out of box, so I have to install b43 firmware first before this wireless card becoming functional. By the way, although Nvidia proprietary driver is close-source driver,which is very bad, it does render graphics far better than nouveau.

I cheered too early yesterday ! Today I installed an input method at this web page: . because this package is provided by a 3rd party(in this case, Chinese internet content provider Sohu.Inc), the installation was completed by double clicking the downloaded deb package and installing it according to its instruction instead of done by using command line “sudo apt-get install …” etc. Then after reboot, I was once again stuck at the SDDM login interface . In fact this issue is neither caused by incorrect grub setup ( nomodeset …) nor by graphic card driver (nouveau or Nvidia proprietary driver), it may caused by the conflict or incompatibility between SDDM and 3rd-party package (in this case sogou pinyin input method). I really like this pinyin input method because I am a Chinese and this input method is developed by a Chinese company. But currently I have to temporarily forget it and use another alternative.

Nice that you solved this issue.
Yeah the nvidia driver usually provides better 3d performance than nouveau.