Out of range

I switched on my computer, started booting up saw the Windows logo and then it went blank then ‘out of range’.
Changed monitors but still the same.
I put the computer together about a year ago and its worked well, just attached a new 22″ tft type monitor, worked well for the last two weeks so can’t understand why it shouldn’t work now.
Please help as kids are now reliant on the machine for homework etc.