Macbook Air cannot detect my wifi I need to factory reset my Macbook Air.
So restarted it and hold option key. 
Then it requires to select wifi.
But the list does not show the wifi I'm using.
The wifi access point is very close and the signal is strong, 
so another Windows laptop and my cell phone are using that wifi.
But this Macbook air shows only other wifi networks and not mine.
What would be the reason?
I tried entering my wifi network's name and password but it doesn't work either.
This might be a case of using channels which are not allowed in your region. The wikipedia page on WLAN channels notes region limitations.

For example, in the US, in the 2.4 GHz spectrum you're only allowed to use channels 1-11, not 12, 13 and 14, while other regions may allow these.

In the 5 GHz spectrum, many countries only allow channels 34-165, or some subset of those (e.g. the US allows 36, 40, 44, 48-116, 132-165). Actually, you'll also note that not all channels in the ranges are listed (e.g. there's a big gap from 64-100, and a gap of 2 or 4 between most successive available channels) so its not necessarily obvious in the 5 GHz range if a channel is excluded in your region.

Anyway, the thing to do is check what region your devices (Access Point &amp; Laptop) are set to, and either adjust them to be the same, and/or set your AP to use a channel common to both regions (e.g. 1-11 in the 2.4 GHz range or 48-64 in the 5 GHz range).