Offline camera
A camera may lose connection because of power, cabling, connectors or equipment failure. Note whether one camera or the whole system is affected and when the issue started.
Missing or broken recordings
Check the recording schedule, date and time, disk status and whether the problem affects live view too. Do not initialise a disk if you need to preserve existing footage.
Prepare for support
Have model numbers, a description of the fault and any error messages ready. Never send your administrator password in an ordinary enquiry. Repair feasibility depends on the equipment and fault.