As far as I know that should be possible for S7 400H CPUs to have RUN/STOP status for each CPU.You can search for H-STATUS blocks in the forum.But for a single CPU, what can be done when a CPU is STOP because there is no usercode execution internally.There are some externalhardware solutions such as pulse monitor for fast reaction to CPU inSTOP.

I supposed you using PCS7 and in PCS7 look for system chart @2, and open it then look for OB_BEGIN block and press F1 on this, help file will give you enough answer. i hope you will find information on this. normally we used this block for display AS status on this.