This is quite a common problem. There are a few things you can look at before get a heating engineer in.

Radiator warm at the bottom but cold at the top

This is usually due to air in the system. Most commonly happens after you've had the heating off for lengthy periods.

Use a radiator key to "bleed" the air out. Unscrew the bleed screw just enough so you hear air hiss out. Get ready to tighten it up as soon as water starts to come out. The radiator should feel warm at the top now.

All radiators warm except one cold all over.

Hot water isn't getting to that one radiator. Check that both the valves are open. Should be fully anticlockwise. Sometimes they are very stiff. If you can't turn them try unscrewing the screw on the cap and pull the cap off to reveal a square shaft. You can get a spanner on that.

Sometimes a valve will seize up so that even though they seem open they are closed. This commonly can happens to thermostatic control valve. These are the type where you can turn a dial to select a room temp. and the valve opens and shuts automatically.
It's a fairly straightforward job to change these valve but you usually have to drain down your central heating system and make sure it's working again afterwards. I would advise beginners to get a professional in to do this.

NB the larger valve is normally used to control the amount of hot water enters the radiator. You can use that to control the radiator temp. The smaller valve is purely an isolation valve and is normally always left open.

All radiators cold?

Is the boiler actually working? Check that the gas burners are ignited.

If you have a combi boiler check that the pressure on the pressure dial is in the range as specified in the instruction manual and top up with water if necessary. (see low pressure on combi boiler)

All radiators only a little warm but not hot enough.

Could be of any many causes. Your need to get a professional in.