Why does my phone keep flashing and not charge?

ok so i have a lg phenoix at&t phone and i havent turned it on for like 12 days so when i try to charge it but not with its original plug in charger but with my usb cable to my laptop cause it used to work and i lost my phone charger.so basicaly it keeps flashing to my start up symbol that say lg so how do i fix it please help