Roblox voice chat stopped picking up my mic

Nine times out of ten it's the input device. Open the in-game menu, go to settings and check which mic it's using, it likes to grab the wrong one after you plug a headset in or out. If that's right, check Windows itself: privacy settings, microphone, and make sure desktop apps are allowed to use it. An update can flip that off. On a phone it's the app's mic permission instead. Also worth a glance at the mic icon above your character, if it's got a line through it you're just muted.