Historically, courts have permitted governments to prevent the spread of disease by quarantining infected or exposed individuals, not locking down whole states. While quarantines are sometimes necessary when instituting them the state should always assess individual risks and consider less-restrictive alternatives.
It is one thing to close businesses briefly to assess and understand a new disease. It's another to paralyze economic activity indefinitely, empowering governors or administrative agency officials to centrally plan economies reopening using arbitrary, unfair, or unequally applied rules. Which is what we have here in Michigan. In our case, it is unconstitutional and the people are making themselves heard about this matter. :)
I'd be interested in reading the court's reasoning.
If a "lockdown" is discriminatory - then of course it should be unconstitutional/illegal. EG: If a state allows WalMart to remain open while mandating the closure of small competing businesses - it is not doing a closure based on health concerns. Plus, the state would Constitutionally owe the small businesses for lost revenue.
Nothing if it's in state court. If it's Federal Court, nothing unless it's appealed to the circuit where it would have impact. If it goes to Supremes, national impact.