brake lever question

Possibly going to sound utterly daft but sod it.

One of the brake levers (flat bar) on my friend's bike is not working - it just doesn't move when you squeeze.
She hasn't used it for a long time, but it's not rusted or anything like that. No obvious cause to look at it.

Any ideas as to what could be the problem?!

(sorry if this has been covered before, I tried UTFS but couldn't find anything)

What doesn't move, the lever or the brake...?
V brake, disc brake, centre pull, dual pivot...?

oops sorry.
The lever doesn't move.
The cable definitely needs sorting out but as far as I can tell that's now not involved in the situation and will need refitting.
And if it should matter then the calliper is a side-pull.

Thank you

Could either be the cable or the caliper: not much else that can jam the lever, as they are pretty simple themselves.

Disconnect the cable from the caliper and try and operate it by hand: does it move freely? Also try the lever: does it work now?

If the caliper is sticky then clean it up and get some oil in the joints. If the lever doesn't move, the inner cable is seized in the outer and both need replacing.

Even if the cable is seized, you should still be able to pull the outer off with a bit of sweat and swearing; failing which get some oil/WD40/Coca-Cola down it and leave overnight.

While you're at it, it would be sensible to replace the other brake cable as well: it'll be in a similar condition and run-of-the-mill cables for both brakes won't cost a tenner.