As per
rules - you're allowed to 'bump' your topic once every 24 hours. But I understand that rule was meant primarily for Marketplace board.
Re-posting the same content could (rightfully) earn you a ban for spamming. It's perfectly normal for topics to slide out of the first page to make room for new ones. You can always rehash old discussion, if you have anything new and valuable to add, just use 'edit' option to update first post + reply with info on the update you've made.
And if your old topic got polluted with idiotic, spammy replies and discussion became impossible to follow - you can open a new, self-moderated one - just add a short info explaining the reason for re-posting.