You may want to ease her into success by reading with her. You can read a portion aloud, then have her do the same. You will quickly note any phonic problems, vision problems, length of attention span and other factors that are important areas for becoming a successful reader.
Keep material that's interesting but easy-to-read around the house for her to pick up in her spare time. A romance novel or magazine might increase her motivation to read. If she flunked a grade, most school systems offer remedial instruction. She should participate in any after-school or summer programs that could be helpful to her.
Another approach to increasing her motivation to read is to have her read to younger children, especially in local library programs or through her school tutoring programs. Many students who tutor younger children gain confidence in their reading and suddenly feel that they can tackle anything.