On Windows:

Wire will ask for permission to access your camera the first time you want to start a video call. Only one application can access the camera at a time on Windows. If Wire is unable to access the camera, check that no other application is using it.

On Wire for web:

Wire will ask for permission to access your camera the first time you want to start a video call. If you have previously denied access, you can update those settings:

On Google Chrome:

1. Click the lock symbol.
2. In the Permissions > Camera section, choose between 'Use global default (Ask)' or 'Always allow on this site'.

On Mozilla Firefox:

1. Click the lock symbol.
2. In the Permissions section > Use the Camera, choose between 'Always ask' or 'Allow'.

On Opera: 

1. From the menu bar, select Preferences > Websites.
2. In the Camera section, check if the option 'Ask me when a site requires access to my camera' is chosen.
3. Click ‘Manage exceptions’. If app.wire.com is set to ‘Block’, click the x to remove the entry.

Please be aware: Camera access can be blocked by Internet security software. Find more information for example here.