My SE was working fine until I took it apart to clean it, and after reassembly the tube won't turn on, what could cause this? enter link description here
Without any other information, my first likely response has to be for you to verify the reassembly. Since I don't know what you actually took apart/disconnected, any errors made putting it back the way it started need to be eliminated first.
If I assume that you did a minimal disassembly of the internals, or have verified everything is back in its proper place (and connected properly and with proper polarity, etc.), then the next thing to check should be the most common causes of a blank screen on compact Macs.
From my experience, the most common cause of this symptom for a previously working machine is a cracked solder joint on the analog board. One of the "features" of these machines was they have good convective cooling because heat generated at the bottom of the machine is allowed to rise and escape through the top, like the cute little chimney that is the original Mac. This subjects the analog board on the left side of the machine to thermal gradients and after so many cycles the solder joints tend to crack from the extra strain, and this can disrupt the video signals.
I've fixed several machines by going over the solder side of the analog board with a magnifying glass to identify any suspect looking joints and reflowing them. You can easily do this with just the case removed and moving the white mylar (or cardboard) sheet that covers the solder side out of your way.