Can be more of one of that two reason, honestly.When posting a thread of troubleshooting nature, it's customary to include your full system's specs. Please list the specs to your build like so:
CPU:
Motherboard:
Ram:
SSD/HDD:
GPU:
PSU:
Chassis:
OS:
Monitor:
Please include the age of the PSU if it's been in use for a while.
A reboot of a system would be either due to one of two reasons;
1| the PSU is incapable of powering your system
or
2| the system is suffering from a thermal issue(overheating