The power come on but
With regards to the blank screen, this usually indicates an issue with the hard drive being faulty (hardware) or the OS might have corrupt system files (software).
These are the items you will need before we begin:
A. Mac desktop/laptop
B. Mac OSX Installation DVD
A. Software Related
1. Power on the Mac computer/laptop and immediately press and hold the option key.
2. After about 15 seconds of holding the option key, release it and insert the Mac OSX DVD.
3. The DVD should load and display on-screen (along with any additional drives available on the system).
4. If the hard drive icon appears, then click on it. (if the hard drive boots the OS, then the issue is software related. Skip down to Section B, step #1).
5. If the hard drive did not appear, then the system does not recognize it. However, the DVD icon should have appeared, click on it.
6. The DVD will boot and begin the installation process.
7. Follow the installation prompts.
8. Once completed, the desktop/laptop will reboot and after logging in, will execute the registration process.
9. Complete the registration exercise and access the desktop.
10. Perform a software update and download all system and application updates to their latest versions.
B. Hardware Related
1. Power on the Mac computer/laptop and immediately press and hold the option key.
2. After about 15 seconds of holding the option key, release it and insert the Mac OSX DVD.
3. The DVD should load and display on-screen (along with any additional drives available on the system).
4. If the hard drive icon appears, then click on it. (if the hard drive boots the OS, then the issue is software related. Restart the computer and skip up to Section A, step #1).
5. If the hard drive did not appear, then the system does not recognize it. However, the DVD icon should have appeared, click on it.
6. The DVD will boot and begin the installation process.
7. Close the installation application and select "Disk Utility" from the "Utilities" menu option.
8. When Disk Utility loads, look to the left-hand pane and locate your hard drive (if detected). If so, click on it and format the hard drive then reboot and skip up to Section A, step #1. If the hard drive is not detected, then the hard drive has malfunctioned and needs to be replaced.
Hard drive's cost from $40-100 for sizes ranging from 40GB to 1TB. Once the hard drive has been replaced, you can install OSX from the installation DVD.