Why in the world did my DVI quit working after I changed out my CPU?

I'm playing around with a socket A Athlon XP system. I just downgraded my cpu from 2500+ to the 1800+ so that I could stick the 2500+ into another box instead. I pretty much kept everything else the same. I changed a few other things, but they shouldn't affect anything. Here are the full details:

1. I fully took out the motherboard from the case to facilitate the cpu transfer (A7N8X by ASUS)
2. I took out the video card, dusted it off with compressed air can, and put it back in (it's a Radeon 8500, 64mb)
3. removed my sound card (permanently)
4. took out my 3 memory sticks and put in 2 others of lower capacity
5. disconnected floppy.

Now, after I put the 'puter all back together, I plug in the DVI cable from my monitor into the video card exactly as before, and it just tells me: "check signal cable". And stays that way when I try booting. When I plug in the analog cable, it's normal. WTH?

I never had this issue before; the DVI had always worked without a hitch as far as I remember. I switched the cpu before (upgraded from the 1800+ to the 2500+... now I reverted back to original setup) and there were no problems. What's the matter now? Anyone have any ideas??

 

Have you tried pressing the 'Source' button on your monitor?

 

Lol. After reading your comment, I was about to try that... I was about to unplug vga and plug in dvi again, and thought, I will make sure to turn off the monitor first. So I did that, finished re-plugging as I was going to, switched the monitor back on expecting to try the source button... but the monitor just came on! And now it works.

D'oh I'm a dolt lol. Don't remember exactly what steps I was doing before; maybe it doesn't work switching the plugs on the fly?

 

It should be able to do so, but some monitors don't agree with that I guess....

 

I'm pretty sure that the very first time what I did was:

1. had the monitor, computer, everything turned off fully. I was ready to power up the pc after doing all that tinkering.
2. I plugged the monitor directly into DVI port (still everything turned off)
3. Then I pushed power button to turn on the monitor ('puter still off).
4. Then it gave me the "check signal cable" or whatever
5. Turning on the computer didn't help -- still no signal.

I don't understand why that would give it trouble. But then it worked suddenly as I described. Doesn't make sense...