Looks like that domain is no longer registered. You have to pay for a domain name each year to keep hold of it. If you still want it, you could re-register it.
Web hosting is usually separate from domain hosting, so it may be that the site is still sitting on your web hosting account is you haven’t also let that expire.
When you’re unable to access it, have you checked with your hosting company that there is no problem with their servers? Does it display an error message, or simply not load?