Hi
Not sure what has occurred here, its always advisable to save your contacts on to your Sim and also Sync to the likes of Gmail if your on Android etc..For this very reason.
I also have my contacts saved to a file on my Laptop, it's safekeeping.
Maybe your Task Manager closed access/use, due to low battery?
My S2 will do this if battery is critically low, all Apps are unaccessable until charge is above a certain amount - but this is due to my deliberate choice of having battery saving mode enabled.
Power off and extract battery and Sim, replace and reboot. Charge up your phone see...
I have never known contacts list to spontaneously Erase.
