I get that sometimes too. I usually can open my eyes with a bit of effort, but if that doesn't work for you maybe just try focusing on the shapes you see instead. They're just as real as the 'world' outside it, try grabbing them through the dark or just look as closely as you can at them to try to find some details to focus on. If you can do that you're pretty likely to have another world appear around it, and it's easy then to forget your eyes were closed at all