Sound Icon & other icons not appearing on screen.

Question
I have 'Acer Aspire 5755 model' laptop.Initially when I increase sound by Hot keys such as
'Fn+Up arrow' etc. system sound get increased with the sound icon on screen.However since some days, when i am trying to use these Hot key combination it is working properly but the corresponding symbol is not appearing on the screen.The problem is same for all other hot key combinations.
Can anybody help me?

You likely have some Acer software that controls whether or not the on screen display appears. Try going to the Windows Start Menu and type Acer to see if you can find the software that controls it.