Know the feeling. I have an Ateo Concepts with the same issue. I have to say I did leave it a touch longer before trying to get the computer working given that it was working previously. The A4000T was the same to be honest. It can get very frustrating. I would always start with the processor board connector to the mobo. It may appear properly seated but the chances it aint. Mine was a pig to get going especially on the 4000T. I kinda dread trying to get the thing working again ... something just isn't connecting sadly and you gotta just take it all the way back and steadily build back up.
I tried various monitors thinking it was that. Also,worth looking at are connectors/ribbons to hard drives. I had an A1200 recently in a tower that had a faulty ribbon connector where the pins had come away and that locked the computer up also. You can forget all the usual tests on these towers sadly.... you have to check and or replace everything one at a time to get to the problem. It'll be a faulty ribbon, connector, SIMM, drive and or a seating issue of the processor. Press down on all the chips you can and even check the seating of the mobo, I had a 4000 with a badly seated board and that stopped the machine working.
Think back if you can to what you were doing when you last were checking the computer over to sell. Were you cleaning anything, reseating anything, removing anything. Think back.
The first row on the SIMM is a pig. I have a 4000 that needs that first one fully seated. Take em all out, check that the connectors are not broken. We have to do all kinds of things with Superglu to get those SIMMS mended. Normally you would get a flashing power light with this issue. Well I did. Could be memory.
Good luck.. PS mine is still dormant here with the cover off. I'll get back to it one day. I'm sure I'll get it working.
http://www.scuzzscink.com/amiga/amiga_scuzz383.htm
