If you feed the starter and it starts to bubble again within 24 hours it is probably still ok. Use your visual senses. A healthy starter should smell yeasty and beer-like. One that is not healthy has a bad odor. After you feed it it should start to bubble again in a day or so. Stir the starter a couple times a day to be sure the ingredients are well mixed and it can continue to process. The color of a healthy starter is creamy.
If in doubt it is not to hard to make up a new starter but it will take about 10 days for it to ferment so you can use it again. A suggestion might be in the future if you make a starter and not planning to use it after it is ready, you can freeze it. When thawed, feed it again and you are ready to go.
Thanks for using Ask an Expert