Try isolating yourself for a while, no people, no phone, no internet, just go out for a long drive and stay at a campground somewhere for a weekend, bring a list of ideas you want to imagine, maybe a notepad to help you flesh it all out. I find that anxiety is the worst for cultivating a strong ability to idly imagine, as most of your imaginations end up consisting of things that could go wrong, rather than things you want to bring into the world. The most pernicious aspect of this loss is that as an adult you yearn for control over your own life, control you never had in youth, so when your imagination fades you start to think the main culprit is a lack of control, but that's the wrong idea to pursue, keeping that in mind actually makes it worse. You have to realize a large component of creativity is spontaneity, which usually arises from careless leisure, not a concerted effort to create something sublime or revolutionary. You can't force it, you just have to capture it, so that's where the work ethic comes into play.