Ally, lets start off with what Apple suggest :"
1. Reset SMC and Clear PRAM to set default startup device to internal hard disk drive (HDD).
User hard drive bootable, issue resolved default settings.
Customer system not starting up, go to step 2
2. Is system indicating a memory error with repeated sleep LED 1 or 3 flash sequence and beep tones if sound is enabled?
Troubleshoot memory issues, go to step 3
Continue with startup sequence verification go to step 4
3. Reseat customer memory and/or swap in known good memory to isolate bad memory and replace defective parts.
Customer memory defective and replaced. Continue to verify startup process. Go to
Should known-good memory fail in one or more slots, replace logic board.
4. Hold the Alt (Option) key during startup and verify there is a bootable hard drive shown in Startup Manager. Choose customer hard drive. Does start up from this drive work?
System starting up from customer hard drive. Startup issue resolved
Customer hard drive not present or does not start up from this drive. Continue to
find bootable device. Go to step 5
5. Insert product OS install disc in optical drive. Install disc is bootable and should be
present in Startup Manager. Can system start up from OS install disc?
Starts up from optical drive - customer hard drive not yet bootable, go to step 8
Computer has no internal bootable devices. Test external startup devices, go
to step 6
6. Boot from a known-good bootable copy of product OS on a USB drive or network
server to start up and verify internal mass storage devices are available using System
Profiler and Disk Utility.
System started up from external device and reports data regarding internal SATA
devices. Go to step 7
No startup devices available Replace logic board" hope this helps, good luck.