The phrase "seize up" typically means to stop working or become incapacitated in some way. There are several synonyms for this phrase that can be used to add variety to your writing or conversation. One such synonym is "freeze," which implies a sudden and complete stoppage. Another is "lock up," which suggests that something has become stuck or jammed and is no longer able to move. "Cramp" is another option, which refers to a sudden contraction or spasm that can cause pain or discomfort. Finally, "halt" is a more formal synonym that can be used to describe a cessation of progress or activity.