When you say they stopped working, do you mean they stopped working to trigger Alexa routines, but you can still turn them on and off by voice through Alexa? Or do you mean they stopped working with Alexa altogether?
Because there was definitely an issue a few days ago with the “virtual switch“ stock DTH, something changed and they are not working the way they used to. Those never did trigger an Alexa routine, but you could turn them on and off and now you can’t always. See discussion in the following thread:
Outage Affecting STHM and some automations (19 Jan 2021)
Or do you mean virtual switches which are also virtual sensors and which you were using to trigger Alexa routines?