H H Yosry,

Did you remove some group or customised any?

Usually, only admin has the access to create/edit the user info. In either case, a non-admin user can belong to a certain group and with the help of right access and record rule, you may achieve this.

Thanks.

No actually i did't change a thing , that is the wierd thing 
And about the right access , every one whom i work with can add a new Users or Modify , 

but latly it was a problem .