With complex lighting the calculation of GI (=global illumination) might take a lot of time if settings are high, or sometimes even at medium settings it can take from tens of minutes to hours. Just wait, or stop it and lower the rendering settings.
It all depends on your machine config too, You did not mention your OS version, AC version or any machine specs?
AC25, Rhino6/7+Grasshopper, TwinMotion • Mac Pro 6,1 E5-1650v2-3,5GHz/128GB/eGPU:6800XT/11.6.5 • HP Z4/Xeon W-2195/256GB/RX6800XT/W10ProWS