Help! Half my house has lost light...

Hi all, would really appreciate some help!
I bought a new light fitting yesterday and put it up in my living room. I stupidly took the old fitting down without noting which wires were going where... there are 4 wires in the ceiling - 2 black (neutral), 1 green/yellow (earth) and 1 black with a red sleeve (live).
I’m pretty sure the old fitting was using only one of the black neutral wires, so I’ve only used 1 on this new fitting (which uses LEN)... however the new light will work, as will the light in the next room, but the ceiling lights in the rest of my flat won’t turn on since I’ve switched the power back on!
Can anyone advise me as to whether I need to use both black wires, try the other black wire or just call an electrician to help me out
Thanks in advance! Annie

 

Thank you both! All working now