To find out what the reason or what cause of this problem, you should first make sure whether this is a hardware damage or damage on the software (operating system). The first step you can do is try to re-install your system. In some cases this can help. However, if the step you have already to do this session, but your notebook always restart again and again then this can be ascertained that damaged is on the hardware of your notebook.
If source of the problem is in the Hardware of notebook, it will be need more specific which where the hardware has a problem, the possibility might is memory, HDD (hard disk drive), or can also module of the mainboard or motherboard of your notebook.
If the hardware problem on the memory, you can try to replace it with newly one or with another memory, or can be tested first with a cleaning up the edge of memory with eraser.
If the problem is on the HDD of your notebook, it is may because any some bad sector on the hard disk drive, or can also because problem on the partition of hard disk drive of your notebook. If the probelm is any some bad sector on the hard drive try to read how to fix and solve bad sector on HDD. If the problem is on the partition of your hard drive of the notebook, try to repartition again.
The Mainboard can also be a cause of the auto restart notebook again and again, if this happens on your notebook, so the problem will become more complex, because the mainboard itself consists of various components. However, the common problem is damaged on the capacitor (sag, drip, that physically can be seen clearly), if that happens, you just need to change the capacitor that is damaged.
If you don’t have any base knowledge and not sure if you can solve, fix and repair your notebook by self, may be you just bring your notebook to the local notebook dealer or the product manufacturer.