You can bump it no problem if it's been a day or so. The issue with bumping occurs when someone does it every 10 or 20 minutes and you end up with a thread with 8 posts that all say "bump" within a few hours.
Posting something along the lines of "Hi, sorry I am not sure how to answer your question, but I'm sure someone will be along soon" is appropriate and contributes to the thread in that it brings it to the top in a more discrete way and shows the poster that someone is reading their thread.