One beep shutdown. My first re-paste. What did I screw up?

So, I’m new here and I posted this to an existing question but thought I’d post as it’s own as well.

I finally got my PS3 slim back after letting my dad use it for a couple of years. It was working fine when I got it back but was very dirty. They smoke in the house and have dogs so it needed a through cleaning, and since it was manufactured in 09, I thought it would be a good idea to re-paste. I have never attempted a re-paste before but I did my homework and it seemed pretty straight forward.
No issues with disassembly, the paste wasn’t completely dry but the board was a little stuck.
I didn’t force it but it did take some time to get it separated from the heat sink.
I re-pasted and reassembled.
Now when plugged in the red standby light come on as normal but when u press to turn on it beeps once then immediately shutdown and goes back to standby within a second or so.
Strangely enough I took the top off last night and went looking for a problem and was able to get it fired up. I signed in to PSN and ran an update and was able to get in a game.
Then I put it back together and it went back to one beep and shutdown.
No matter what I try now it’s just one beep and off.
Anyone know where to start looking?