Video conferencing platforms require audio and/or camera permissions from your device in order to function correctly. Upon first opening such a platform, your device should prompt you to allow any required permissions.
Crossing from one platform to another may cause permissions issues on your device and as such, you may encounter a loss of audio or camera functionality. If your device doesn't prompt you to allow these permissions and you are encountering issues, restarting your device may resolve the issue.
If it doesn't, you will need to contact support for the platform with which you are having issues.