Agreed with GilaMonster, blindly shrinking a database might not be the best way of reclaiming space.

For this particular issue. If you are in a rush to get the server back, you could detach this ReportServerTempDB, move it to another location, attach it back then do your investigation or wait until your DBA comes back.

Also, the default recovery model of this database is SIMPLE, no need to change it unless it has been changed by your DBA. In this case, you might want to check with him/her.