Temperature danger zone 40 f to 140 f. Needs to be kept in fridge or else kept above 140 f in order to be safe to eat per public safety standards.
Depending on how good your immune system is you might be fine if you eat it. The safest thing would be to heat the meatloaf up to 165 f for 15 seconds, which should kill any foodborne pathogens that grew while sittin out all night. However, I've heard of some heat-resistant strains of micro organisms that could survive being heated that hot. So, YMMV.
<-- used to be serv safe certified, but certs expired in '07.