The Domain Name Service or "DNS" is the Internet's Sat-Nav.
It translates friendly and easy to remember names like bbc.co.uk into a less easy to remember numerical "IP Address" like 151.101.192.81.
It knows the best "route" to get from your PC to the online website or service you are trying to access.
From time to time, some Service Providers might have problems with their DNS, and the route to certain sites cannot be found, where others can. Users of a different service provider might have no such issues and can connect to all the sites and services that they would usually access.
Some years ago, we had an issue where BT Business broadband users could not access Caterbook (and Twitter, and Lloyds online banking) for several hours, but regular, "domestic" BT Internet users had no such problems.
There are workarounds if this happens, but unfortunately they require a degree of technical ability. Starting with the easiest first:
1. Try connecting your PC to a phone's mobile WiFi hotspot.
It needs to be a different service provider from your main "office" broadband.
2. Flush your DNS.
From a command prompt, type ipconfig /flushdns hit the return key.
It's a long shot, but it might just grab the most recent DNS routing records, and replace any older and now "fixed" issues.
3. Update your DNS settings on your local PC, to add in Google's DNS servers 8.8.8.8
You will need the admin password for this if there is one. In Windows, click Control Panel. From Network Connections, right click your currently active connection.
Click Properties. Scroll the list of services until you see Internet Protocol Version 4 (TCP/IP v4).
Click to highlight this and then click the "Properties" button. Add Google's 8.8.8.8 as your Alternative DNS server.
(Probably worth flushing DNS as per 2. Above.)
4. Update your hosts file.
Find Notepad. Right click and Run as Administrator. You will need the admin password for this if there is one.
File -> Open. Navigate to C:\Windows\System32\drivers\etc
Select the file "hosts"
add in the following line:
51.141.90.252 api.caterbook.net
Save.
(Again, it's probably worth flushing DNS as per 2. Above.)