Is it illegal for spouse to change locks on house while still married?

In this instance I have to assume you have an attorney handling the divorce. The best advice I can give you is to consult with them as they know the specifics of your situation and will know what to tell you to best help you with the following proceedings

However in short most states do not allow for a person to change the locks on a home owned by a married couple while in the middle of a divorce. Assuming that neither of you has been given sole rights to the property at this point it is still owned by you both and she should not have the right to change the locks and refuse entry.

However because there are many other possible factors at play in this case before making any decisions or taking any action you really need to find out from your lawyer what they would advise based on your specific circumstance

If the house is in both your names and there's no legal order preventing you from entering it--no restraining order, no order in the divorce proceedings giving her sole right to the house, etc.--then no, she can't keep you from entering. The house is still your property. However, to avoid the situation from possibly escalating, you want to secure your right to enter in the proper, low key and legal way. If you don't have a matrimonial attorney who could advise you, try contacting your town or city police and asking them for their advice on how to resolve the situation. They may be  able to help you gain access in a way that doesn't turn it into a confrontation between you and your wife, or at least could refer you to the appropriate agency to help.