This has been known to happen for at least two reasons.  One reason is that the app can glitch when the WiFi connection changes from one server or network to another, and another reason is that Siri encounters a processing error.  Turning Airplane mode on and off will reset the WiFi connection and the app should begin translating again. If this does not work, then activate Airplane mode and then turn off the device. Turning it back on and deactivating Airplane mode should resolve the glitch.