Please help. After I erased Windows 7 using another PC and the Acronis program installed by the previous owner of the laptop, my Macbook stopped responding to various combinations to install MacOS. On 4pda, I was advised to first download the Ubuntu image, and then MacOS Snow Leopard. I tried to boot from both Ubuntu on a flash drive and MacOS Snow Leopard. All the same, first the power-on sound, a white screen, then a black screen and a message, like on Windows computers - "An operating system wasn't found...." etc. I changed the disk, but a line pops up saying that there is no boot disk.