which is correct

[chochokeemalyia]

Why network is not working and since when is it not working?
Why network is not working and since when it is not working?

[Casiopea]

Try,

Why is the network not working, and when did it stop working?

[jack]

I'm learning english myself. I hope this helps:

...when is it not working?
...when it is not working?

1. When it is not working, you can trouble shoot the system. (Use 'it is' for statement.)
2. When is it not working? (Use 'is it' for questions.)