how to resolve unmounted drive

Hi I have a QNAP TS-233 whose status is unmounted. It was working reasonably well until recently – even after I had to change one of the 2.0TB drives with a suitable replacement. However, now it is showing as in a unmounted state. I read your article How to Fix Unmounted Raid but could not discern whether your instructions would be suitable for my model

Please link to the ‘article’ your refer to.

what i not understand