java.lang.NoClassDefFoundError: - AndroidStudio

I have a DbAdapter and a DbHelper class into my android app, to handle login and register requests through SQLite, they were working perfectly, until now (maybe a Clean Porject or Rebuild) generated this error, maybe a database reload? I don't know.

This is the actual error if I click on a button in my app (Login):

06-26 12:22:08.164  28592-28592/com.example.shairlook E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.NoClassDefFoundError: com.kkoci.shairlook.DbHelper
        at com.kkoci.shairlook.DbAdapter.open(DbAdapter.java:38)
        at com.kkoci.shairlook.LoginMember.onCreate(LoginMember.java:57)

This is the error code on DbAdapter on line 38:

 public DbAdapter open() throws SQLException
{
    mDbHelper = new DbHelper(mCtx);
    mDb = mDbHelper.getWritableDatabase();
    return this;
}

And the Login Member Activity:

        dbAdapter = new DbAdapter(this);
        dbAdapter.open();

If I click on Register button it's the same error, same line on DbAdapter and different lines on, Register Activity of course:

    dbAdapter = new DbAdapter(this);
    dbAdapter.open();

If you need further explanation please let me know...

Any ideas?

Thanks in advance!