Add something that will create some ammonia in the tank. Some people add some flake or frozen fish food, some add a small shrimp. Personally, I had some PURE ammonia be sure there is no surfactant added... it will be listed in the ingredients label which you want to say 100% ammonia) and added a small amount to the tank, waited 30 minutes and tested for ammonia. I got a low number so I added a second dose and 20 minutes later I had a good ammonia reading. The foods will decompose over time and you will get an ammonia spike that way as well, it just takes time. The ammonia level will go down and nitrate goes up. Then it will go down as nitrate will go up. Unless you do something the nitrate will remain in the tank.
At least that way you will be sure you have enough of the right bacterium to keep your tank moving forward. Given you may have good live rock, I'd add ammonia and test after it's had a chance to spread through the tank. You may already have the bacterium you need and the run from ammonia to nitrite to nitrate may happen over just a day or two. If your LR isn'r so 'alive' it may take the tank as long as a few weeks to run the process.
Then do a water change to reduce the nitrate and you are safely on your way.