You may have seen and installed v1.2 of the Signup Form Plugin. In testing there were a few situations where we saw the plugin break after an upgrade. If that happened to you, please check out the upgrade notes and try what they suggest before posting.
The back story here is that we've brought on CrowdFavorite to maintain and continue to develop the plugin for us going forward (with some direction from us, of course). In doing that, they fixed a lot of non-standard/old things with how the plugin worked, which is what has caused some of these issues - that's also why it was bumped from 1.1.x to 1.2. While that may sting a little now, as they continue to improve the functionality and general WordPress compatibility, everyone should be much happier in general.