Why the contacts would suddenly disappear is a mystery. Most contacts either sync locally and with an email or with Outlook. Check and ensure that they're actually there.
BB10 has this new thing called "LINK" which is really nice but also really confusing. I can't copy linked data to my PC for some reason if it wasn't originally there or set to be saved there. Retarded IMO. Anyways - check that.
Knowing the solution doesn't mean knowing the method. Yet answering correctly and regurgitation are considered "learning" and "knowledge".