Computer starts but no picture

I have a Dell Optiplex 740 (which used to run Windows Vista but at some point has been upgraded to Windows 7), it belonged to a friend of mine and after a period of time of the pc not being used we wanted to get it going again. It is an ageing pc, it has 2009 stamped inside, and has only been sat around for a couple of years.I connected it up this morning and turned it on, I can hear its on because the fan is quite noisy and the green light is on, there is also 2 numbers in green at the bottom that lights up, a 3 and a 4, no idea what that means. But the problem is I don't have a picture, the light on the monitor keeps flashing, which normally means no connection. If I unplug the monitor the light stays on and it tells me to check the connection, but plugging it back in the light flashes again. The computer worked fine when it was last used, any ideas as to what may be happening?

Those are diagnostic light indicators. From the Diagnostic Lights section of your User's Guide here.

That error seems to indicate you either have a floppy drive connected that has failed or checking for a floppy drive was turned "On" in the bios and getting the error because there's not a physical drive present. Probably turned on by default and turned off at some point but due to a CMOS battery being low the bios reset to defaults and of course turned that back on.

There's no floppy drive present, I don't think it has ever had one. It is very possible the CMOS battery is low, especially as the computer hasn't been turned on in a while, so that sounds like the most likely cause. I'll look in the Bios and change the settings to stop it searching for a floppy drive, and hopefully then we should be all sorted.

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Is all sorted now, changed the settings in the Bios and is all now up and running, thanks everyone for the advice.