HMRC may accept a reasonable excuse such as serious illness, an unexpected hospital stay, bereavement, fire, flood, theft, or a major disruption to its online services or the post. You should explain what happened, provide evidence where possible, pay the tax as soon as you can, and appeal the penalty through your HMRC account or using the relevant form. Simply not having enough money is generally not a reasonable excuse, although exceptional circumstances may be considered. Check the latest guidance on GOV.UK, as HMRC decides appeals based on the individual facts.