If a timing belt breaks, would the lights/door locks etc still work or it would be absolute silence/no lights when you go to turn the key?

You would still have everything electrical working except for the alternator which of course would need the motor running to work. Charge your battery with a portable battery charger set to 12 volts, 10 amps, for 6 hours. Then start the vehicle and drive straight to an auto store and have the battery and charging system tested for free. I'll bet the alternator is bad. You'll only have around 30 minutes of drive time if the alternator is not charging the battery. Jumping the battery won't work if the alternator is bad and the battery is dead. The motor will die when the jumper cables are removed.

List the make, model, year and miles. That is key info.

In general the electrical should work when a timing belt fails.

Are you sure the timing belt failed?

When a timing belt breaks, it would not prevent the battery from powering everything the battery typically powers.

If the timing belt breaks, the car will not start or run, but as long as the battery is charged, all the electrical stuff will still work.