Availability of the website is a very unpredictable thing. Sometimes from home (or work) your website is accessible, but your friends can't enter it. Also there can be a situation when sometimes your website up and sometimes down (and you can't even prove this). Or website is being loaded very slowly. Solution in each case can be complicated, but let's consider the basic types of such problems:
Unavailability of a large segment of the network. In this case, users from one network segment can't get access to websites in the other segment (eg, due to routing errors). It's easy enough to install special utilities, but you can't usually perform anything in this case. And the problems of this level (global) typically affect hundreds of thousands of sites, and are being solved them quickly.
You can only sit and wait. And hope that your users are in the same boat (in the same segment), which is your website.
Unavailability of a hoster (small segment of the network). In this case, the problems usually lie on the side of the provider (and therefore can be corrected not so fast). Accurately set the frequency and nature of these problems manually is practically impossible, but it is relatively easy with tools for automatic monitoring.
There is necessary in case of any downtime greater than 10-30 minutes to write to support of your ISP: maybe they do not know that something has fallen off?
Great timeout. In this case, in fact, the site works. And open to all users. But not all users can wait for it :)
It is also possible to check (server timeout with WEBO Pulsar, and the total download time with webpagetest.org. If you page generation time on the website is too large, you should apply any of optimization techniques. Otherwise there will be big losses among target users.
HTTP-error on the site. Quite often it happens that due to heavy load (or service provider hardware failures) website starts with an error (500, 502, 503 or similar). Tracking of such things and in-time moving the website to a safer place is also possible, but this of course requires special attention.
WEBO Pulsar would be the best for this purpose (to track when, what time, and with what frequency). With it all unprepossessing "kitchen" ISP come out and you'll know exactly what is happening with your site.
The failure of the site. Sometimes there are minor incidents with the website (due to hardware failure or hosting provider network configuration change), when the availability of the website is not directly affected, but part of the website functionality is broken. For example, database queries are no longer handled. Or debugging messages have been appeared.
Complete knowledge of such situations can only be possible with direct viewing the website (or its basic functionality check). With WEBO Pulsar, you can check how the presence of a specific text on the page, and its absence. This enables the most timely response to the problem.