You can contact HMRC's self-assessment helpline by calling +44 300 200 3310.
You will need you UTR number and National Insurance number, which you can find on your profile.
If they tell you to register for a personal tax account, and you found it too difficult, then be sure to tell them.
You will need to know what tax year you need the records for (this will be something like 2016/17 or 2017/18).
Ask for the employer name, PAYE reference number, gross pay, and tax deductions.
Remember that it is your responsibility to have made sure you received this information, and also to have kept it safe. They may not be to be willing to help you at first, so have a bit of patience and be nice, they are doing you a big favour!
Once they agree to send you the information, you should receive it within 2 weeks. Once you have it, you can login and add it to your account.