How do you know?
It’s not possible for you to manually access your website every minute of the day and night to make sure it is up, but there are services that will do this for you.
A website is no use if it is down and unreachable by potential clients so website monitoring is an important addition.
My own website is monitored by monitor.us:
It’s a free service and will send you an email should your website ever go down.
There are also other additional benefits from the service, that enable you to monitor the site and its performance.