If you have to solve a cryptex puzzle and have difficulties opening it, you can click on "solution" (you will be penalized, but it will allow you to continue). So as to find the solution out, you should have a look at all the items you have gathered in your inventory so far. You can do that by clicking on the folder icon (bottom of the screen).
Tip: the correct answer is number Pi, but remember to round the figure to get an exact number! Good luck!