My display unit of my ACER PC show no signal,but it is on. what can be the problem?

'No signal' warning shows if the monitor is unpluged from the cpu, but in your case i dont think its the reason (still verify the back cable of the monitor and cpu, plug it tightly) If you were using your system fine and suddenly one day it went off to 'No signal' warning then it might be the problem in the ram. Mostly cleaning the ram will fix this problem. Follow these steps carefully:
-open the cpu cabinet,
-remove the ram from the slot (if 2 rams then remove both)
-take a eraser (rubber) and rub the surface area (copper lid area) gently both the sides
-nor place the ram in the slot in right position
- close the cabinet
restart the computer

please rate and leave a comment. thank you!

First shut down you computer remove the covers of the cpu then locate the memory and remove them wipe them clean with clean cloth and switch the computer on with out the memory and see if you hear any beeps if you hear beeps insert the memory and switch on the computer if still no beeps then remove the processor fan and clean the surface of the processor fan and the processor apply a thin layer heatsink solvent and clip the processor fan tighty to the socket