Twice this evening my PC has closed itself down and then restarted.
Could this be a sign of something failing?
Overheating (fan failure or excessive dust) or a device driver bug are the main causes of unexpected reboots. If the machine does not want to reboot, overheating may be the issue. If it reboots with no problem, it's more likely to be a driver.
Check that all fans are running. Blow out any accumulated dust. Make a note of what you are doing when the machine reboots.
Failures caused by drivers are almost always caused by video drivers, which always contain bugs. Updating the driver may help, although in the case of video drivers you are often just trading one set of serious system-crashing bugs for another.
Sometimes a failing power supply, motherboard, or CPU may cause unexpected reboots.
If the reboots increase in frequency, it's probably hardware. If they do not increase in frequency, it's probably a driver.