Why would the reverse gear not engage all of a sudden but the rest are working? on 2002 Ford Focus

Problem just occurred today, difficulty getting reverse to engage, then after several tries it went in. However, it has not engaged again today with approx. 20 attempts. No unusual noises heard. What are all the possible problems? How can I be more detailed? Is there a diagnostic tool for this?

Asked by for the 2002 Ford Focus
get free diag and est from aamco then shop prices
agree with greg