If you're talking about the standard box sash window, it's probably the temp/humidity change of this time of year, and the frame has swollen a bit. You can either wait, or start prying off the trim around the frame (the part that prevents the moving panel from falling into the room!) From there, get a craft knife and gently slice down the gap between the frame and the window to get it moving. Once it's out, spray with silicone spray, make sure the weights on both sides of the window are still tied, and then like the Haynes manual says, refit is the reverse of removal

Is the outside of the sash painted?

If it 18C outside why the hell you want to open the window, that'll let the cold in.

Oh and only an american/canadian could infer that a 30 year old house is old. We would consider that a new homw over here.

18C cold?

Jeebus man! That's tanning weather in Canada!

You must be in the UK or jamaica if you think that's cold..............

Try rubbing a candle inside the tracks. It won't help initially opening it, but the more open it gets, the easier it will slide. Once fully open, rub candle in lower section of track. Open and close a few times to get even coating of wax.

If you can't get it to move at all, take a block of wood that will fit in sash channel and lightly smack it with a hammer a couple of times on each side.

Edit: I have heard baby powder will work instead of candle wax, but I have never tried it.