This code is not readable. Create a list of roles, and assign users to roles via a joining table. Make the enum a bit field, and store that in the session. Having '1 is managers' is an arcane rule that makes your code hard to maintain. Don't store the username and password in the session, why would you do that ? Use switch instead of if/then/else, and ideally switch on method calls, so your code is readable. Like this
switch(userType)
{
case User.Admin:
LoginAdmin(username, password);
break;
case User.Basic:
LoginBasicUser(username, password);
break;
}