|Why did you change the mboard ?|
Was it because it wasn't working because of any of these things ? :
Have you, or is it possible someone else who had access to this computer...
- dropped it, or dropped anything on it, or otherwise exposed it to a physical jolt ?
- spilled liquid on it, or sprayed it with liquid, or exposed it to liquid such as rain ?
Since the computer last worked properly all the time, has there been a power failure event that happened while the AC adapter was plugged in ?
Those things are the most frequent reasons something inside the laptop has been damaged.
Did the original mboard have a password that was set in the bios that you didn't know ?
If yes, was that password for hard drive access or system access?
Are you using the original hard drive, or one that was in a different brand name computer ?
Did you install a mboard that's identical to the original one, the same HP part number ?
Are you sure that that mboard has no passwords enabled in the bios ?
Searching the web with
"serial not found" HP notebook
"serial not found" HP laptop
doesn't find a lot of "hits" but most seem to be only for a problem with HP computers.
Are you sure it's exactly "serial not found" ?
If the more to the error message ?
If yes, qiote the entire message.
Try using an external USB corded keyboard.