iMac G3 Bondi Blue black screen problem

Revcently I bought an iMac G3 Bondi Blue (Rev B) in pretty good shape. I decided to downgrade it to its original specs.

I swapped the hard drive (a 20 GB hard drive) with a 4 GB Hard drive and I also replaced the system battery.

Finally I cleaned it inside with compressed air.

After a hard time closing the iMac (I had trouble to fit the transparent bottom shell) I started the iMac.

I heard the start sound and the display showed me the file folder asking for a system disk. I turned the iMac off and restarted it with the original Mac Os 8.5 CD.

Now, the monitor was blank, but I could hear the start sound. I tried it several times but the monitor stays blank. Also, the USB mouse does not show the typical red light (I use an old Apple Pro mouse). There seems no pwer in the mouse or in the keyboard.

Any ideas?

Does it have to do with the new battery or the hard drive?

Thank you!