When I turn on my computer I get a grey screen for a few minutes until a blinking folder with a question mark in it appears. Often, my computer shuts off after the question mark folder disappears. I have read that booting from a disk is how to fix this problem, but I am unable to do it. I've never tried before, but I have been unsuccessful. Any ideas?

The blinking question mark means that you macintosh can't find a bootable disk. It has nothing to do with the VGA connection. More than likely, your directory structure on the disk is bad. An excellent product to fix this is Disk Warrior. It has saved more hard drives for me than I care to think about. Worth every penny.
As far as booting up from a disk, just put in the CD and hold down the C key until you see the spinning icon.
You could also try these to things if you don't want to purchase disk warrior, but I don't think they will work. Hold down the shift key while you boot until you see the apple logo. The shift key is to boot it in safeboot mode. Or, you could also reset the pram:

1. Shut down the computer.
2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
3. Turn on the computer.
4. Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.

The question mark appearsof your computer that means your vga connector problem
maybe not properly connected try to unplug & replug again
vga connector that connecting from the cpu to monitor or try to use other vga connector