Some snippet managers will show you a dialog box with a helpful error message, like the one below, but others do not.
One of the apps you have open has enabled secure input, which means Dash is not able to expand snippets or search using selected text. This usually is not a problem (apps enable and disable secure input all the time). However, if you are not able to expand snippets or search using selected text at all, it means one of your open apps enabled secure input and forgot to turn it off.
It’s easy to overlook the name of the app in the dialog box, so look carefully. Simply closing the app or saving the item being edited will allow the snippet manager to start working again. In the case of the macOS loginwindow I find that locking and unlocking my Mac will clear the secure input status.