There are a few methods to try to change the password when you don't know the QSECOFR password or when the account got disabled. The easiest way is when there is a duplicate profile of QSECOFR with the same power. Then when you lost the password or the QSECOFR got disabled you can still acces the other profile and ue the CHGUSRPRF to set the user to enabled or you can change the password in case of a lost password. Offcourse in your case the probability of a duplictae user and the ability to log in to that one if it even exists is very small.
It seems that you have no option of a second profile so you need to use the dedicated service tools menu (DST). Using those dedicated service tools you can reset the password. The menu can be used during a mnual IPL. The steps to take are slightly different depending the OS/400 system (eg V5... or V4...) and you need to know the box. Is it partitioned, non-partintioned etc.
For non-partitioned boxes go to manual mode using control panel. For partitioned boxes you need to sign on to your primary partition and then change your partition startup mode to manual using the system service tool menu STRSST and the system partition option. Then IPL (Initial progrma Load, what we would call 'boot' with a pc). The system will show the DSP menu, there for system V5R1 choose option 5 dst environment. The DSP log in appears, there log in with the DST security profile, this is a DST ID of the SECOFR, the default password for the DST QSECOFR is QSECOFR. Then select option 6 security tools from the DST environment menu, and then choose the option reset operation system default password. (for V4R5 you need to choose the option DST user profiles and then reset system default passwords.) After doing that the password can be reset to it's default only for this IPL session. Go back to the DST menu and choose option 1 to IPL your system. When the system comes up change the QSECOFR back to enabled when it was disabled and change the default password in one of your one. Then put the IPL options back to normal mode. But like said in the above post if you have no clue then you better don't do the things described above.