Continuous beep tone from my CPU for nothing

Hello Everyone,
Recently I wanted to switch on my PC. My system boots with beep tone but nothing appears on the screen.
After that I tried to clean up my CPU and I cleared all the dust on motherboard and checked all connections are OK.
Clearing the graphics card I resolved my issue. But this time after about 20 minutes I heard from my CPU continuous beeping and my monitor became darken.
My PC is running smoothly but there is still continuous beep tone and my monitor light is very low. I tasted my monitor. There is no problem. What can I do now?
