My Account not visible in User Accounts

Castis' answer did not quite fix the issue for me, even after a full restart, on Ubuntu 16.04. After editing /etc/login.defs to change UID_MIN and GID_MIN from 1000 to 500, I could use adduser to create new accounts with low UIDs, but my old account (with UID 502) was still not being shown.

It seems that AccountsService was caching the fact that it once thought this was a system account. To fix this, I had to manually edit: /var/lib/AccountsService/users/<username>.

In the [User] section, I had to change SystemAccount=true to SystemAccount=false

After restarting the AccountsService daemon (sudo service accounts-daemon restart) my username reappeared in the System Settings->User Accounts GUI.

Notably, this was an account that was carried over from a previous install of 14.04, and had had its uid manually changed.

I was unaware that there was an account manager running. I solved this issue by editing /etc/login.defs, changing UID_MIN from 1000 to 500, and restarting.

You could probably get by just sudo service lightdm restart but I restarted the machine as I was also performing a distribution upgrade.

In my case for some reasons my account disappeared from the User Accounts GUI. Checking the answers above I managed to do the following steps:

1. From terminal I changed directory: cd /var/lib/AccountsService/users/
2. Then I opened up my account there (check it with ls) with nano: sudo nano <username>
3. Then I changed the SystemAccount property to the opposite value (like from false to true).
4. Saved it and opened up again and changed the SystemAccount value back to it's original.
5. Restarted User Accounts GUI --> Woala it's working again.