Are you using 2 screens and are using the Mixer or Editor on one of them in fullscreen mode? If your project becomes unresponsive at start up or when trying to export, the problem might be the following:
A pop-up alerting you of missing files or plugins / the export dialogue box has opened behind the detached Mixer/Editor but takes priority over it. Studio One becomes unresponsive until you close the window, but you can't see it or access it with your mouse as long as it's hidden. Here's how to get the window back in view:
1) Press Alt+Space
2) Press M ('Move'); Your mouse cursor turns into a four-headed arrow.
3) Use your arrow keys and navigate the hidden window to your other screen to interact with it.
That being said, I'll actually turn this into a feature request: PreSonus, please make dialogue boxes like that appear IN FRONT of detached Windows like the Mixer or Editor. I nearly lost my mind trying to figure out how to "rescue" my project; There were two missing plugins and not even restarting with VST-Support disabled allowed me to avoid the pop-up informing me of those missing devices; disconnecting the second screen didn't do any good, and the Mixer and Editor positions are saved with the project, so no matter what, it would always open in this way and block itself immediatly. It shouldn't be this hard. ;)