This has happened a couple of times but this is the only time I managed to see what the cause of this problem. Before this when I leave my computer overnight and when I wake up the system already not responsive I'am not been able to turn wake my monitor from standby.
This time I just go out for couple of hours and when I get back my computer already become sluggish. So a quick check with System Monitor I found out compiz is using 2.4GB from my total memory of 4GB and 100% swap was used.
Anyway picture worth a thousand words. So here it is.