If you are not able to login into your plesk control panel, please refer the below,

/usr/local/psa/admin/logs/httpsd_error_log

If you are getting the below error message, it means you are using more memory than alloted to you,

[emerg] (22)Invalid argument: could not call shmget

In such a case you need to increase the value of shmall in /etc/sysctl.conf file. Edit the file and increase the value as kern.sysv.shmall=shmmax/4096

If you need the changes to get reflected soon, please execute the following command :-

sysctl -p

You need to reset the psa service, in this way you can access your Plesk control panel.