There was a bug in a previous version that could cause the sampler lens to get stuck on in some odd situations. We added some code to prevent this happening and most reports of it occurring have gone since the 2.1.4 update. However, if you're using 2.1.5 that makes 2 reports of it happening with the current version. I believe it's a timing issue that can be exacerbated by not having clean resources on the iPad (eg. lots of apps running in the background and no recent reboot) but we'll look in to it further. It is probably triggered by tap and hold colour sampling - If you use the colour sampler tool you can turn off tap and hold colour sampling in the preferences panel and this may help while we investigate. If you're not using 2.1.5 please make sure you have updated.

Update: I forgot to add before: If this happens again, try tapping the Menu button on the toolbar then closing the menu to see if this fixes it. The freeze that was happening isn't a freeze as such, it's just the canvas not responding because it believes that there's already an operation going on. Opening and closing the menu may solve it if it happens (or at least allow you to save).