There's a few possibilities. Firstly, be careful handling live HT leads and plugs - the voltage is very high and you'll get quite a jolt if you form part of the ciircuit.
Check the earth - normally a braided strap from the engine to car body.
At the distributor, check the points are set okay and there is a spark - easier seen in the dark. Also if you have a distributor cap, check its condition and that the contacts for the leads aren't wet or full of oil.
Clean all connections at earth, coil, distributor, leads, coil and plugs and check no wires are damaged.
Also check the battery is well charged.
If the battery is okay, remove a spark plug and place the spark end of the plug in contact with the engine so it makes good contact. Making sure you are clear of fan belts etc, get someone to turn over the engine while you are watching the spark plug. It should produce a blue spark. If no spark, or a weak yellow spark, then there's a problem with coil, rotor/cap, condenser, leads or spark plugs - swap them with known good ones, one at a time until you track it down.
Hope this helps.
