Window doesn't react during calculations

Question

Hello,
I am writing a program at the moment which is able to solve a certain set of differential equations for subsequent time steps.
I would like to implement it in such a way that one can press "start" and it will start the calculation timestep by timestep until it reaches some max number of steps, however, I also want to implement a button called "stop" so it can stop the calculations at a time decided by the user.
At the moment I don't know how to do that, because during the calculations the window just doesn't react and if I try to click something it crashes (so i guess it is stuck in calculations).
Is there a way to overcome this somehow?

Cheers,
Phil