Desktop PC (Chalfont St Giles HP8)

Non-working. This was a refurbished Dell Optiplex 790 which served me well for a couple of years. I switched it on after a week away - lights were on but screen was blank. I changed the CMOS battery but made no difference. I've removed the SSD hard disk and replaced it with a reformatted mechanical disk (working). It's got an i7 processor and 16GB RAM but no OS. Needs to go to someone who knows what they're doing.