Certified GKH Gardening Expert
If it's wood ash, it's OK, but charcoal is not a good additive. However, wood ash should be scattered lightly, not dumped in a large pile. It's preferable to compost it first. You may want to test the soil pH. Wood ash can raise the pH; that may be why someone suggested adding sulphur. It usually takes some time to change soil pH, so I doubt the wood ash has made a big difference.
If a large amount of wood ash was added, it could add lye and salts to your soil when it gets wet, which can burn plants. Small amounts are not harmful, such as when lightly scattered. But privet can tolerate some salt.
Charcoal has additives that leave a chemical residue, which can be detrimental to plants. You can try to rake out any charcoal bits you see, then add compost over that and work in lightly. It also may be time to fertilize your privet. The article below tells how.