If you were successful in updating another app and only suffering for one app then this may be the case of no storage available on your phone to install the update, try after clearing the space. Or due to updating another app you may have finished your data so, the app is stuck on pending, check your data and then try. Now, if the app which is stuck was on the starting or middle of the update queue (if the internet connection is fine and there is sufficient storage on your phone)  then it's a serious issue and you should contact the developers to look after it. To contact the developers scroll down on the app screen and click on the Developer contact button and write an email to them regarding the issue.