Screen went black on laptop, worked on external monitor for 3 days, then it went black too after I opened the back of laptop to inspect the CPU card. Hard drive has been checked & works good on another laptop, but won't work on my laptop (HDD light does not come on & won't spin). SD card light does not come on when SD card inserted. DVD spins when inserted. I replaced CPU card & no change. I think I might have damaged the motherboard somehow, but don't know how to check it. HELP?