Black screen

Hello,
switch on computer and all usuall signals show but only for one or two seconds.then screen remains black.when i swith monitor of and on again screen lights up but again for only a few seconds then goes black again.i keep switching on and off at mains and eventually i get full screen display,this process however seems to be tacking longer and longer as each day goes by...is it my computer or is it just that my monitor is just packing up..?