Law enforcement sources tell us the "Pretty Little Liars" actress' San Fernando Valley home was broken into and burglarized very early Friday morning. We're told the perp smashed through one of her front windows and stole jewelry and purses currently valued in excess of $15k -- but that number is likely to go up once full inventory is taken.

Lucy was not home at the time of the break-in. We're also told her alarm was on, and all signs point to this being a random hit ... not a case of burglars targeting a specific celeb.

That jibes with the string of burglaries over the last year -- which, as we've told you, cops believe are being carried out by professionals looking for nice houses. Celebs just seem to be casualties getting hit along the way.