Macbook to second monitor thru vga stopped working

once i found out it was possible, i bought a vga adaptor for the mini port in my 15' macbook and connected my old monitor and even my tv sometimes. but i didn't do it for a while and then i decided to set it up again. i plugged everything in and the screen when blue (as usual) for a second on my screen, showing that it recognised that it was plugged in. but on the monitor it said no signal. i went into displays in system preferences and usually it would show it was connected but it was just normal. i changed the actual vga cord 3 times and still nothing !! does this mean i have to buy another adaptor or can someone tell me how to fix it ??

thanks guys

You're not using any graphics switching programs like gfxCardStatus are you? Also what if you untick the mirror displays option?

You did change the input on the monitor/TV to the VGA input?

Unplug and reinsert the adapter?