No signal when turned on

Not good... sounds liek the motherboard has gone up the creek , try removing the left side of the case , remove the little round battery on the mother board , take it out for 30 seconds then put it back in and try start the computer then

Hi! Odhran
Not good to mee too, may be is your motherboard problem, or, just reset the CMOS on MOBO, be carefull on that, like Furion187 say, and try to startup your PC, if not work, you may have a power supply problem, time to change, or your RAMS are bad. take out one DIMM and try to restart, or if you only have one DIMM put it in other slot.
Good luck!