No one can agree on what this song is about, though many have tried to analyse it.
This is my 'take' of what I think is going on in this song.
I hope you like it!

This was UK band, The Zombies 1st Single.
The Zombies recorded this in ONE TAKE after they won a talent contest at their college called the 'Herts Beat Competition'.
The prize was a recording session.
The band also recorded a cover of Gershwin's "Summertime" for their first album, which was considered for the band's first single, but "She's Not There" got the nod. Boosted by radio play on New York Powerhouse WINS, the song became a hit in the US.
Some of the chord changes Argent used were inspired by Brian Hyland's "Sealed With A Kiss."
This was The Zombies' biggest hit.
Even though it did very well, their next releases didn't catch on until 1969, when they hit US #3 with "Time Of The Season."
Unfortunately, the band had already broken up by then and Argent had started his own group.

Keyboardist Rod Argent wrote this and most other Zombies songs. He later formed the early 1970s group 'Argent' with Zombies bassist and co-writer Chris White.
Lead singer Colin Blunstone re-recorded this in the early '70s under the name Neil MacArthur. His version went to #34 in the UK.

The lead instrument is an electric piano, which was rare at the time.
In the UK, it was the first hit song with an electric piano as the lead instrument.
Rod Argent, the keyboard player in the Zombies, was challenged by Decca Records producer Ken Jones to write a hit record for his group, after they had just won the Herts Beat talent contest and he came up with this.

Santana covered this on their 1977 album Moonflower. Their version hit #27 in the US and #11 in the UK; it was the only non-live song from the otherwise live album.
It was the last Santana cover song to chart.

Images from Google
Music: THE ZOMBIES