A Dedicated Denial of Service (DDoS) attack happens when a server or network receives a huge spike of incoming traffic, enough to exceed its ability to handle requests in a timely manner or to handle them at all. Imagine filling a water bottle using a firehose, that's sort of what a DDoS attack is like.
This most commonly happens with websites and personal networks. If a website's server is DDoSed, you'll find that if slows to a crawl or stops loading at all. Personal networks are often attacked by edgy teenagers who got 360 no-scoped in Call of Duty and want to take out their anger on the pro gamer who did it.
If you are connected to Windscribe, you'll find that you don't actually have full protection from DDoS attacks. The attacker would need to send more traffic as our server can handle more than your private network, but if someone finds your VPN IP and traces it to our server, they are still able to attack it with a traffic spike. It wouldn't even be the DDoS itself that causes it, in most cases the server host will just null-route all the incoming traffic in order to protect their infrastructure.
To add protection against this sort of attack, we would need to make our service less private. It would require additional info about where the traffic comes from and/or a system to distribute that traffic in a way that would not be feasible with our service. Therefore, while highly unlikely, there can still be a DDoS attack on our servers which would cause your VPN to disconnect. Your private network would be unaffected though and chances are, the VPN will quickly reconnect to another server at the same datacenter or a different datacenter nearby.