I agree with Joe’s reply : if they’ve just purchased, there are several companies who’ll offer free equip and installation in return for a monitoring service contract of a year or more. Otherwise, identify original manufacturer and either Google to source spare parts needed (if they know what needs repaired) or call the original manufacturer for service/service provider options. Good luck!
I would recommend contacting the existing security system company first. They usually have contractors onboard with material who is familiar with common issues relating to their systems.
If service repair "is" available, I would contact the original equipment manufacturer's (OEM) website to see if that is an option. The technology in security cameras and systems change rapidly, so the camera and/or system, may be outdated. Replacing the units may be a better option. Today's NVR systems have much greater resolution and night vision clarity.