Anyone?? Please!? I am desperate here for any help!

Hello, I am new to the site and I in need of some help **PLEASE** I have recieved a letter this morning re a compliance check and I need to send in reciepts/ contracts etc. I have been STUPID- my daughter turned 4 last March and got 15hours free at her daycare. I signed a form re this and assumed this would inform the tax credits that my costs had decreased. I was claiming £160 per week when my costs were £70 after the 15 houts free. So i have been overpaid and never rang up the tax credits to ask why/ change the details... I just thought that automatically happened after I signed the 15hour form. Anyway I am going to send all my contacts/ money paid to them and inform them of the overpayment it appears I have. My questions are-has this happened to anyone before? What was the outcome and will I be sent yo court/prison? I guess my overpayment is around £5,000 which I know is a massive amount. I am grateful for any advice/past experiances..anything really. Please help! Thankyou in advance.. you have no idea how sick I feel xx:sad: