My PC reboots while playing osu!
This is usually caused by your graphics card or CPU overheating, or some other hardware fault.
To determine the cause more accurately, download a GPU/CPU temperature monitor and keep close watch over your hardware while playing osu!. If your hardware is regularly reaching temperatures of >90C / 194F, you may wish to try enabling the 120FPS/240FPS Frame Limiter in the game's options, as this will significantly reduce the load on both your CPU and GPU.
Alternatively, setting your CPU/GPU fans to 100% will help dissipate the building heat quicker - there are several applications which will allow you to do this, such as MSI Afterburner and SpeedFan.
If overheating is not the issue, you may wish to investigate your sound card - specifically whether its drivers are up to date or not, along with any peripherals you may have plugged in to your computer.
If none of this helps, please send in a support ticket to firstname.lastname@example.org with more information on the issue - including your computer's technical specifications, model (where appropriate) and any other information you think may help us discern the cause of your problem.