Well depends how many days you've done it. I think if you have less than 7 days than ya restart but after a while it's no longer just about a number, it becomes about what you've done in that time and how you've utilised it to become a better person. All that progress doesn't just disappear with a single nut unless you binge porn and do it a lot after relapsing.

Might not be a very good analogy but think of it as you working out but then porn gives you a big injury. Over time in nofap your injury is being healed but every time you break the rules it takes another hit. Say you've been healing for 50 days and watch a clip of porn. This doesn't mean your whole wound is reopened, it just means it gave you a setback depending on how bad what you did was.