Sometimes it is up and sometimes it is down.
Nobody ever explains why, but it's about as predictable as the weather.
It can get so infuriating you want to throw the whole lot out of the window - or sit down and cry. The truth is, basic networks can be made to function so reliably they only go down once or twice a year - and those times should be scheduled in advance. Most networks are shaped like a star, with one or more servers at the centre and cabling tentacles reaching out from there to each work station or network client. You have to convince the network server that you aren't an impostor. This means giving it your login name and password correctly, and not just trying to bluster past this point if the machine says you have got it wrong.