This depends entirely on the severity of the compromise. In general, honesty is your best bet, but you also don't want to cause unnecessary concern, so you should be careful to very clearly define what, if any, impact is possible to the users.
In general, people are bad at understanding technology and even worse at understanding security. Make sure you clearly explain what, if anything, your users need to do to protect themselves if anything may have been compromised. Don't rampantly speculate, but also don't fail to warn them if something is a possibility of impacting them.
If your passwords were anywhere near compromised, be sure to invalidate them and send user's notification as many users will have used the same credentials in more than one place even though they should not do that.
If it was a very minor breach, it might not be necessary to notify users at all, however if it could have impacted users in some way, let them know so you have a second set of eyes looking out for any other problems that may have occurred. It is also wise to make sure you consult with a security expert to verify the risk to your users in the likely event that you are not a security expert yourself.
Note, it may also be worth asking about this question with some more details about the actual breach over on IT Security where people are a bit better equipped to point out any possible security implications to your users that you haven't thought of.