There are two ways you can answer this type of question. One is to acknowledge that you have experienced a network crash and discuss how you recovered from it. The second is to state that you never experienced a network crash, but then describe what you would do if one occurred. This is an operational question, and the interviewer is seeking to understand how you go about doing your job. Describing the correct way to respond to a network crash, either historically or in the future, will accomplish this.
"Unfortunately, in one of my previous positions, we did experience a crash of the network. This was due to a denial of service attack from an outside hacker. The first thing we did was to immediately shut down the entire network. We then discovered the port which the hacker was using to infiltrate the network and closed it. We reset the DNS server and tested it to verify that it was working properly. We then brought the network up, one segment at a time. We were satisfied that we had solved the problem and that the vulnerability had been fixed. The entire process took less than 3 hours. This was not optimal, but it was better than being down for days or having our data compromised."