With any device problem like that you need to look in Device Manager before posting so you can post what you see in there, such as any driver error symbols, unknown device etc.
It may be you just need to re-install the
Crystal Eye Webcam driver which you can download from Acer's support website. Drivers for UK laptop and PC models are available here:
http://www.acer.co.uk/ac/en/GB/content/drivers
For other countries, click "United Kingdom" at bottom-right of page.