It depends on what's causing it. This could be a symptom of many things, including ADHD, bipolar, depression, anxiety, psychosis, dissociation, pretty much everything under the sun.

Sometimes under stress people have trouble thinking. If you're having problems with anxiety or racing thoughts, you can try meditation. Keep track of your breathing and keep it slow. You can try grounding-- go through each sense and describe to yourself what you see, hear, and feel. You can create harmless sensations to help you focus by using a strobe light (be careful if you're photosensitive), listening to music, or holding an ice cube.

If you're calm and still having trouble organizing your thoughts, try writing them out/journaling. I have disorganized thoughts sometimes, and it helps to write out what I'm thinking because I can edit and correct myself when I get off topic or get confused. You can also take your time, unlike with talking or just thinking to yourself.