1. Overheat due to too much dirt. Solution - vacuum clean the interior. You may need to remove CPU fansink and clean the dirt with tissue inside the gaps of heatsink.
2. Power Supply Unit failure - Replace power supply unit
3. Motherboard failure - Check for swollen capacitor, it is a sign of motherboard failure, but not a necessary symptom.
'No signal' is shown when your monitor is on but it cannot detect any signal from your PC (because your PC is off). This means the PC cannot be switched on at all. This is either due to a faulty power supply or a faulty motherboard. Temporary workaround is to shut down the power completely. (Because for ATX power supply, there is still standby power even the computer is off, and there is still electricity in power supply and part of the motherboard).