The ? mark means the machine cannot find a valid System Folder to boot from. Does this machine have a CD drive? Did you get an OS 7 or 8 CD ROM with it? Presumably it has a hard drive in it - this would indicate that either the hard drive isn't working, or more probably, the System on the drive is borked. Unplug the machine, open it up and make sure all of the internal cables are secure. You might also remove the RAM and reinstall it. THe machine definitely might need a new PRAM battery, it is a 3.6V 1/2AA Lithium battery, availabe at Radioshack etc. for about $8.
You can download System 7.5.3 here
http://download.info.apple.com/Appl...System/Older_System/System_7.5_Version_7.5.3/
(disclaimer, I have not tried this install personally since about 1997)
You will need 19 1.44 Mb floppy disks and a Mac with a floppy drive to do this.
Alternatively, look on eBay or your local used computer / Mac store to pick up a copy of OS 8
Your machine could go as high as OS 9.0
http://docs.info.apple.com/article.html?artnum=25114