I agree with @SomeoneElse that more info is needed from you for a real answer. But here are a couple possibilities that I can think of:
You’ve made a change that’s considered an “error” by the iD validator, such as adding a node or way with no tags. You can’t submit the changeset until that’s resolved. Try checking the issues menu on the right ( icon)
You’ve accidentally undone any changes you made, so there’s nothing to submit. That would make the save button appear “blocked”.