Bios not starting on power on/restart

hi all :)
need a little advice on when the machine is turned on there is no output from the bios to start the boot-up process its happened about 4 times in the last month the fans/lights are running in the machine so its powering up ok just no BEEP and boot up it has done it today on a normal restart after updating some programs (urging me to post it in this thread) so i power off the machine and unplug the mains input and press power button for 8 secs to drain out power from psu/mboard and re-power up and the BEEP is there and bios kicks in and windows starts ok. now on my own research and looking at the problem i might be looking at having to replace the battery on the motherboard, but would appreciate anyone's feedback and advice many thanks brian

Does the screen appear? Is it just the Beep sound that isn't happening? If so, it could be that the connections to the internal piezo speaker have become loose.

the beep sound sounds and machine boots ok normally. but 4 times over a month when the machine is turned on there is no beep and no boot up just blank screen but fans are spinning etc and on a re-boot today it happened

i was due to clean the inside of machine soon so i could check connections on the drive etc over the weekend is it worth repairing the mbr with the repair disc?

When it does boot is the time in bios and windows correct? Normally they won't be if the the mother board battery is bad. That being said; those batteries are cheep and easy to replace and reset the time in bios and Windows.

yep checked the time all is correct

machine is two years old now over the weekend i will shut it down and check everything connected and give it a good clean inside i will change the battery and see how it fairs and report back to the thread thanks for all your input :)