If there are no signs of life in your sourdough and it's not bubbling or rising, even after you have kept it in a warm place, it probably just needs more time to get going. Give it another refreshment. If it did show signs of life and then appears to have 'died', it may be that it has become too acidic. In this case, discard all but 130g of your starter and give it another refreshment. If these steps don't work, i.e. if you still don't notice any life in the sourdough at all, then start again from the beginning with a fresh source of flour.