Tear into it and re-make connections. If no go, try the HDs in an external case to see if they can be accessed.
I have an old Dell Desktop 8200 Pentium 4, running Windows XP Home Edition.
I once dropped the pc 5 cm, after that no possibility to start up again.
Message: Primary disk not found
Secondary disk not found
Tried starting up from cd rom, that doesn't work either...