This is one of the very common types of attacks done on a network. Because of a DoS attack the network / service is unavialbale for use.

Denial of Service (DoS) Attack:

A denial of service (DoS) attack occurs when a user or organization is deprived of the services of a resource they would normally expect to have. Typically, the loss of service is the inability to access a particular network service, such as email. Although usually intentional and malicious, a denial of service attack can sometimes happen accidentally.