[i have crabs]

i doubt the waterchange caused a cycle, did you test for amonia and nitrite? if you had a spike in amonia then yes to the cycle being a posibility, it would have to be kinda major to be killing stuff and i thing something else probably happened.
how about some tank info, size?, skimmer? type of filtration? sand?how old is the tank?
same goes for the cyano, access your tank situation and make a plan or figure out a possible cause, cyano wont grow without food, so there are probably ways fix the cyano with out sticking a bandaid over the problem.