Fat PS3 shows red light for standby mode, but won’t turn on?

Hi! I’ve just fully cleaned an 80GB ‘fat’ PS3 CECHL03. It previously worked but was very dusty. After cleaning, it turns on as far as standby mode showing a red light, but will not power on beyond that, it either shows a green light briefly then powers off, or just powers off immediately. It did not show yellow light. Does anyone know what might be the issue, bearing in mind it was powering on correctly before I gave it a full internal clean? (i.e what could I have messed up when reassembling?) Thanks ?