Some children have a hard time getting started on a task. They sit and stare at the worksheet or book and do not know what to do. There are several things that may be going on.
They may know what they are supposed to do, but they do not know how. In this case, the child needs additional instruction. You may need to work with them to help them understand how. Maybe reading the previous pages in the book to them will help. Or, they may have some notes or worksheets that explain how. If that does not help, you might need to call their teacher to let them know the problem.
They might know how to do the task, but they don't know what the task is. It is very likely that they did not read the directions, so they do not know how to get started. I like to have them read the directions out loud to me. Then, I ask them to tell me what the directions say. Sometimes that is all the help they need.
They may be feeling overwhelmed. If you feel that they do know how to do the task and they understand the directions, try covering up everything except for one problem. Ask them to do that one. Then uncover another one. Sometimes this will alleviate their emotional block and allow them to get started. Experiment with how much you need to cover up to get them moving.
Very often children can tell you what is happening in their brain. It never hurts to ask, "What is wrong? Why can't you get started?" They may just be daydreaming about an ice cream sundae with sprinkles on top!