Hex projects have underlying kernels, which are essentially programs that run your code.
The kernel status is displayed in the upper-right of the Logic view, and in most cases should be green or "Running".
In some cases your kernel may display "Stopped". You can run a cell or select the Run All option to get the kernel going again.
You may also occasionally want or need to restart your kernel, which you can do from the Run All dropdown. This is the equivalent of restarting your computer, and it can fix an unusual amount of problems.
Restarting the kernel will erase the stored value of all python variables. If you have irreplaceable data stored in a python variable that you know cannot be reproduced next time you run the app, you should save that somewhere before restarting.