Is there a way to tell that the qubes system has been hacked or any other system and what are the ways to restore the system to default? as in other OSes it is not possible to do so.
It is very, very unlikely that your dom0 got hacked unless you installed something in dom0 which was specifically designed to “hack” your QubesOS.
You can install annoying and malicous browser extensions in a browser in one of your qubes. If you do this in a dispVM you can switch off the annoyance quickly unless you entered your name, phone number or place where you live (or credentials, credit card number, and so forth). But this will not compromise dom0. You can run purposefully malware in dispVMs (linux and windows alike). To compromise dom0 that has to be an extremly targeted attack of someone. Please, if you know someone who does this kind of stuff, I’m starting hiring in January.
You can throw away the compromised appVM and add a new one using the same template.
The best way to do this for the entire system is to perform a clean reinstallation and optionally restore using “paranoid mode.”