How to enable a disabled User Account on a MacBook Pro running OS X El Captain?
Usually the proper disable Login Shell value is: /usr/bin/false instead of /bin/false. So I don't know if your /bin/false is just a typo or not.

Even using the false UserShell I haven't been able to reproduce your problem though.

You can try the following: open Terminal and enter

dscl . read /Users/user_name


to check if the user user_name is still there.

Then check UserShell:

dscl . read /Users/user_name UserShell


To modify UserShell use:

sudo dscl . change /Users/user_name UserShell /bin/false /bin/bash 


Depending on the value found previously you may have to use /usr/bin/false instead of /bin/false as shell which should be replaced.

Then recheck UserShell:

dscl . read /Users/user_name UserShell


which should yield:


  UserShell: /bin/bash


Exit Terminal and open System Preferences. The proper UserShell value should make the lost user visible again in the System Preferences > Users &amp; Groups.