The notion of thawing a frozen food and then re-freezing it afterwards is often a grey area for home cooks. Generations of kitchen lore has decreed that it's a bad idea, raising the possibility of foodborne illness. In truth, foods such as spaghetti sauce can usually be thawed and refrozen without any ill effect.
Hot and Cold
The safest way to thaw your sauce is in the refrigerator. It can take overnight or even longer, but that way the sauce remains at a food safe temperature throughout the entire process. You can heat only what you need and refreeze the rest, or heat the entire batch and freeze the leftovers. The sauce must be brought to a temperature of 165 degrees Fahrenheit or higher, like any other food that's being reheated, to kill any bacteria that might be present. It must be cooled and repackaged for freezing within two hours of being taken from the heat to prevent bacteria or mold spores from becoming reestablished. The sauce will freeze most quickly and retain the best quality if it's packed in shallow containers or bags.