It's quite common to get stuck on a problem--often because you have initial ideas about what the solution should be that block your ability to see the real solution. Which of the following are a good next best step to take when you've already spent time reanalyzing the problem by focusing intently, and you find that you are simply stuck?
1) Keep working away on the problem--the solution will come as long as you keep your focus on it.
2) Stare intently--it is your focused mode that will provide the big picture perspective you need to solve the problem.
3) Switch your attention to something completely different, or even better, go for a walk or take a shower--anything that allows your mind to relax and dart randomly around.
4) Mentally rethink the problem yet again from where you started, following each step carefully to look for where you may have gone wrong. Even if you have done this several times before, it might work this time.