If you have just recently changed the IP address associated with a domain/site on the server, then your website might be inaccessible momentarily.
Changing a site's IP address may cause it to appear down from some locations until the DNS has propagated to all nameservers on the Internet. You can check if you are experiencing some DNS propagation issue by using online tools similar to this:
Or by running a DNS query against a public DNS server and comparing the result to the IP address that you have assigned to site's virtual host:
dig +short DOMAIN.COM @188.8.131.52
If the IP does not match the IP that you see in Apache's configuration file then it's more than likely that you are experiencing some DNS propagation issues.
You simply have to wait until the propagation is complete and the DNS value resolves to the correct IP. DNS propagation can take up to 72 hours to propagate worldwide, although it typically takes a few hours, so please allow sufficient time.