Over the last few days HMRC have been sending letters to self employed taxpayers which look like this
If you've received one don't worry it isn't the end of your world. By law, HMRC have to tell you that you must submit a tax return for the year ended 5 April 2018.
As long as you complete your return and submit it before 31 October 2018 (if it's a paper one) or 31 January 2019 (online) you've done your bit.