1
Boot up your laptop, then repeatedly press the F12 key and from the resulting menu, select Diagnostics.
Run the full Dell Diagnostics test, does that indicate any problem with the hardware?
2
If not, look at the function keys (F1 - F12) on your keyboard, if one of them has a camera icon, press that key or Fn + that key to unlock the webcam.
2
If that was not the problem, look at the camera lens above your screen, is there a small privacy slider that needs to be opened to access the webcam?
3
If the webcam is still not working, go to the support page for your laptop on the Dell website, then from there, manually download and re-install the version of webcam drivers they provide.
https://www.dell.com/support/product-details/en-ie/product/xps-13-9315-laptop/drivers
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Standard Disclaimer: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.