If the server suddenly stops responding, there may be several reasons:
the proxy stopped functioning;
the provider has blocked access to this service;
on the computer appeared malware, which changes the network settings, including the proxy.
The first thing to do is try another proxy address. Anonymous servers are very many, the provider will not be able to block everything, just like all services will never close. For good tips you can try https://pro-papers.com/technology-writing-service .
In the event that several of the proxies you choose refuse to work, or worse, you do not use an anonymous server for surfing on the network, but you get an error in the browser "Connecting to a proxy server is not available" - this means the work of malicious software. It is programs, because antiviruses can often miss such a threat. Specialized programs are needed to identify and treat this problem. For example, Hitman Pro is a free scanning utility for searching, detecting and fixing malicious software. Ideal for securing the antivirus and diagnosing the system in real time.