When I upgraded from Lion to OS X Mavericks from the App Store, it completed then I restarted my computer and then it said…
Please enter your system pin lock to continue
Or something along the lines of that, but I don’t remember ever setting a System Pin Lock though and I am quite confused as to how it happened and now I am locked out.
If your Mac is used or purchased second-hand from a retailer other than Apple, then you were most likely locked out remotely via Find my Mac.
Consider three courses of action:
Since several people on the Apple site have reported that they are locked out of accessing their Mac, go to the nearest Apple store and have them unlock it for you.
Contact the previous owner (or reseller) of the Mac (if second hand) and ask them to unlock the Mac via Find my Mac.
Access your iCloud account and see if you can unlock it from there.
Those are really your only courses of action.
You can try reinstalling the OS from a CD or flash drive it only wipes global system settings and does not damage your user account files.