First you need to open the Login screen by clicking the 'Login' link from the main page. You'll be asked for your username and password; type these in the textboxes and then click 'Login'. You might have to wait a few seconds, but you should then be logged in and presented with the main ByteHoard interface.
If your login doesn't succeed, then remember that passwords are case-sensitive; make sure that any capitalisation is the same as before and that your Caps Lock and your Num Lock (if you are on a laptop or using a compact keyboard) are both off. If you still can't login, you might have to ask for your password to be reset.