Can my employerstop paying their portion of my health insurance premiums whileit temporarily shuts down?

If there is a written contract, including a union contract, specifying benefits, then the terms of that contract must be followed. Without a written contract, the issue will be whether or not there was an implied employment agreement that the employer would pay 90% even when closed. An implied agreement can be created by other, noncontract documents or communications setting out terms and conditions of employment (such as an employee handbook) or *sometimes* by behavior--i.e. by what the parties (the employer and employee) have done in the past. However, since the general rule is that employment, in the absence of a written contrat, is "employment at will," the usual default presumption is that employers can change what they pay or offer as benefits at will going forward. (They can't retroactively change it for time already worked.) It usually takes strong evidence to show a limitation on this power owing to an implied contract; and conversely it takes very little to preserve this right--for  example, a simple disclaimer in an employee handbook can preserve the employer's right to change benefits at will.

Therefore, it is *possible* that the employer may be unable to do this, but it is likely that they can. Employers generally have enormous freedom for setting employee vs. employer contributinos. If you think there might be an implied contract, you should consult with an attorney who can evaluate the sitation in detail. If there are a number of employees in the same boat, you might retain the attorney jointly to spread the cost. Good luck.