I actually did this exact thing on Saturday morning.
Digging through the freezer on Friday night, set pack of mysis cubes on top to get to something else. Forgot to put them back in. Saturday morning, notice thawed pack.
Chucked it.
When I come across situations like this I just ask the question of myself "is it worth the risk of hundreds, perhaps thousands of dollars in livestock, time, effort and headache to save $8 in fish food? Brings things into perspective for me.
It might be just fine to use 99,999 times out of a 100,000. Do you want to be that one that it went bad for?