Can't unlock LG Optimus P500
All you have to do is HARD RESET Your Device.

Before it,try wrong  pattern continuously for 10-15 times .It will show some option where u can unlock your screen with your Google play account.

If it doesn't work try hard reset.  

For hard reset


Switch off your phone.
press power button+volume up/down(try both separately) for 10-12 seconds.
It will show several option .
choose the appropriate one (don't choose options which you don't understand)


for selecting the menu press 'menu' button or try some other buttons if it doesn't work.
the menu choice depends on the different versions of Android phone.