PC Power Problem

Question

My Advent T9103 has for some time been experiencing problems on Start-up after being shut down the previous night. It would sometimes take a long time to Start-up when pressed momentarily and sometimes I would have to depress the Start button for a long time before it would start. As a result of this we decided to leave the PC on permanently. Unfortunately, the PC shut itself down the other day and we have not been able to start it up again since. Have you any advice please?