Hi there.
Remove all power to the desktop, remove the side panel, power the machine up and check that CPU fan is working, if it is turn the computer back off, get a clean, dry paintbrush and gently clean the fan blades and the metal heat sync behind the fan of dust, then restart the PC to see how long it stays on for. If this time it stays on it was overheating.

Good luck and thanks for using Fixya!!!