Yamaha HPDI 300HP Problems

[Kipstone]

Hi All
I have problems with my 2006 HPDI 300HP on my 1976 rebuilt 23 Seacraft. Ive recently purchased the boat and only used it a couple of times after winter storage. The tanks were 3/4 full. On first lengthy running of 30 mins at 3,500 the engine lost power and would only rev to 1500 but no warnings went off so it did not go into safety. I just got home but then would only start to tickover and cut with any throttle. Now it will not start at all. Ive cleaned the main and engine filters and checked the plugs. Any ideas? Ive seen lots of issues on Forums but just help in identifying the problem would be of great assistance. Thanks in advance

PS Also thanks to those who gave me great advice on my recent trim tabs question