If you are self-employed or have income which is not taxed at source you will need to complete a self-assessment tax return. A tax return will need to be submitted to HMRC for the period 6th April 2015 to 5th April 2016 and can be prepared and submitted any time between 6th April 2016 and 31st January 2017. However the sooner you prepare and submit the information the sooner you will know how much tax and national insurance you will have to pay by 31st January 2017. 
HMRC will send you a notification to submit a tax return if you have registered with them. If you are no longer self-employed or do not have any untaxed income then you must advise HMRC. Don't ignore the notification to submit a return because it doesn't apply to you any more as HMRC will not know that and will fine you. And they won't retract that fine.
Last year I was asked to help a young apprentice who had been fined for not submitting a tax return. She may have received a notification but doesn't remember and anyway would have dismissed it because she only had employed earnings. I helped her write to HMRC and we rang them too but they were adamant a return was due. They passed us from one department to another and nobody could explain why one was due just that it was. Eventually we found out it was because she had a P11D benefit and HMRC had been unable to recover the tax due through her tax code because her earnings were too low. So instead they expected her to complete a tax return declaring the benefit. They then expected her to pay for the benefit in one go as well as the £100 fine they had imposed for not completing the return! Madness! Why didn't HMRC send her a letter explaining why she needed to do a return? It took 18 months to get to the bottom of the reason why she received a fine.
So many people ignore correspondence from HMRC. Don't ignore it. If you don't understand what you need to do or why you need to do it ring HMRC - there will be a contact number on the letter.
