20l still can be rooted.use this.
http://xdaforums.com/showthread.php?t=2112190
however,f180s 20g can't be rooted.damned lg fixed this backboor again.
all right.press volume - and power button till you see some words on screen.when you see them,release those buttons quickly.then press power button to confirm.after do this,your phone should work again.
so,you maybe need to flash kdz file.
http://csmgdl.lgmobile.com/swdata/WDLSW/LGF180L/ALGTBK/F180L20l_00/F180L20L_00.kdz