MacBook Pro won´t let me log in after PRAM I was having a problem with a pointer so after installing latest system updates with no effect on crazy pointer, I did a PRAM reset. 

From then on, a log in page has changed: 


my user name has changed (in past i was using my whole name as login name. Some months ago, I changed it for a 'nickname'. But now, after PRAM reset, the old/previous user account name appears.
there was another account 'guest' not secured by password. Now the password is required.
'my account' with changed user name doesn't accept my valid password. When typing a password incorrectly for 3 times, there is pop-up to log in with my apple ID. That does not work either. The password is correct. 


I am using this: 


MacBook Pro 2011 late
latest operating system with updates
I never changed my apple ID since created
changed my Apple ID last year
changed my computer password also about a year ago
It seems like the main issue to address here is being able to log in again with a valid password. Once you've done that the rest can be sorted.

Since you've got a late 2011 MacBook Pro, you should be able to use these steps to reset your password from Recovery Mode:


Power off your MacBook Pro
Boot into Recovery Mode by pressing the power button while holding down CommandR
If prompted, select your language and press enter
Choose Utilities > Terminal.
Enter resetpassword (all one word, lowercase letters)
Press Return
Select the drive with your account (typically the main drive)
Choose your account using Select the User Account.
Enter a new password 
Re-enter the same password
Enter a password hint
Click Save 
Ignore the warning about the password changing
Click OK.
Shutdown the Mac using Apple > Shut Down
Restart your MacBook Pro normally and use the new password to log into your account


Let me know if this doesn't work.