I am from Dell social Media and Community.
I am sorry to know about the webcam issue.
This is one of the unusual issue that I have seen. This could be a possible software issue with the system, as the camera works for one click and does not work later. If this was a hardware issue, the webcam would not work at all.
I would suggest you to try and perform a system clean up, by using an Anti-Virus application and verify if your system is infected.
If your system is clean, perform a System Restore, to a previous date when the system was working fine without any issues.
If the system restore has been disables and you cannot access it, the only solution I would recommend is to back up all the important data from your system and perform a factor reset.
Let me know if you need any further assistance.