Try to debug your code line by line (F8) and you'll see why the Form2 is not accessible.
I suggest you to change code like this (it will be more readible for you):
If txtUsername.Text = String.Empty Then
MsgBox("Please enter your Username", MsgBoxStyle.Information, "Login")
Exit Sub
End If
If txtPassword.Text = String.Empty Then
MsgBox("Please enter your Password", MsgBoxStyle.Information, "Login")
Exit Sub
End If
If txtUsername.Text = "admin" And txtPassword.Text = "admin0123" Then
Me.Hide()
Form2.Show()
End If
If txtUsername.Text = "stacey" And txtPassword.Text = "stacey01" Then
Me.Hide()
Form3.Show()
End If
Hint: Storing the names of users (logins) and passwords inside the program (inside the procedure) is not good practice.