Black video display - Gateway Solo 2200
 

My wife received a cast-off Gateway Solo 2200 laptop without a power cord.
It came with two batteries, floppy drive, CD drive, and master 2G hard drive.

She bought a new power adapter. but we cannot get a picture on the screen.
We tried using an external monitor (pressing Fn+LCD/CRT) with no difference.

The system seems to respond to commands (I booted up an old DOS disk).
I failed to open it (to check for loose cables, and to reseat any loose chips).
I cannot see to change the BIOS settings (to enable booting the Linux CD).

Any idea what to try next, or should we just return the power adapter?

You might want to check inside to see if there is anything obviously wrong with the computer that you can fix, such as the loose cables or chips that you mentioned. You might also want to see if the owners manual has anything to say about this. If neither of those fix the problem, then, there is a good chance that some hardware is bad and it would be best to return the power adapter.