Frozen meat should be completely thawed before you start cooking it. This is to ensure the centre (which is frozen the most) reaches the right internal temperature for cooking without overcooking the outside.
Source: Safe Handling @ Meat Safety
- Is it bad to cook frozen meat without waiting it to be unfrozen? @ StackExchange
Excerpt: To cook turkey properly, it must come to an internal temperature of 180. If the meat is frozen, it is going to take a lot longer for the internal temperature to rise that high, so the outside of the bird will be somewhat overcooked (compared to roasting a thawed bird). That meat will be much tougher than it would be otherwise.