Phone has died and won't turn on.

yesterday evening my phone, which has been working fine, died. I last looked at it about 7pm and it had 60 or so % battery. When I went to bed at 10pm I checked it to see if I needed to charge it overnight and it was dead. It would not turn on. I left it on charge overnight and still no joy. It has not been in contact with water - it was simply sat on the sofa next to me. It does occasionally turn itself off but I can always turn it straight back on. I have had it two years in May.