why is my mac led flashing every second when booted up?

Hi, I own a mac mini leopard 10.5.8 and I use it for editing youtube videos for my brother. I recently just opened my mini to clean all the dust out, and after i put it back and plugged it all in to test if it works, (only power cable) the power LED on the bottom right flashed every second with no pause. I tried reseating the RAM like someone told someone else to do with the same situation but that didn't work either. I bought this thing somewhere in the 20th century but i've taken the best care of it.this is about the third time i've opened it for maintenance throughout the years and it's never done this. Any suggestions?