I guess try a different sim card first to confirm. It might just be your sim card pin is not correctly sit.

Have you ever disassembled your N900? I did mine countless time.
Remove Kernel Power and revert it back to stock, you should be fine after that. Otherwise it could be that your device OS is corrupted. Reflash will fix this.

There is nothing needed to be done with your new computer. Just download flash maemo, then download the image file. Run the commandline and put the device into flashing mode then you're done.