Can I refuse to let my child go with my ex husband for a weekend visit?

Technically, you are in contempt if you do not comply with the court order. However, the fact that it has been so long may give you a reason to file for a modification and request a temporary order for a step-up plan. This may depend on how old your child is and whether there are any safety or other concerns other than dad's absence for this period of time, as well as what dad's reason for being absent (moved out of state and moved back, for example, may be more excusable than if he were incarcerated or something). You should visit with an experienced family law attorney in your area about your specific current Decree/Parenting Plan and the specific facts of your family's situation before determining your options.