While your monitor should be able to communicate its' abilities to your Raspberry Pi, there are times that the 'standard' that an old monitor uses is not recognised by the Pi. This is especially true with old VGA monitors for which you need to use an HDMI/VGA adaptor like the one available from The Pi Hut. The Pi tries its best, but sometimes you have to intervene and make some manual changes.
Of course, if your Pi is not displaying the image correctly, how are you going to make those changes. Fortunately, the boot partition on your SD card (assuming you are using Raspbian, and not NOOBS) is readable on a PC or Mac. On that partition is a file called config.txt which sets up your Pi as it boots.
First, try uncommenting the following line by removing the # from the start:
This line tells the Pi to force output through the HDMI port, and not the composite port (the yellow plug on the original Pi, or shared 3.5mm audio/video port on the A+/B+/Pi2). Save the file, eject it from your PC/Mac, put it back into your Pi and try booting again.
If this does not work, try uncommenting the following line:
This will boost the HDMI signal. The value ranges from 0 (the default) to 7 (maximum). Try increasing it if necessary.
The last value you may try to uncomment is:
If one of these three works, you may find that your Pi is not using the monitor's optimal screen resolution. If this is the case then you may have to manually tell the Pi to use a specific resolution.
Once again, edit the config.txt file and add the following four lines:
The first line tells the Pi to ignore everything that the monitor is telling it about the resolution it can run at.
The second line tells the Pi to output video and sound through the HDMI port. Most VGA adaptors have an audio out for connection to your monitor or external speaker.
The next line tells the Pi to use a resolution defined in the 'DMT' group of resolutions supported by most monitors.
The last line tells the Pi which resolution to use. Mode 28 is for 1280x800. The following lists some of the most common resolutions:
There are a lot of other resolutions available if yours is not listed above. A full list is available on the Raspberry Pi website documentation area.
Hopefully you now have a working VGA monitor!