The question mark means it can't find the ddrive it thinks it's supposed to boot from, most common cause is drive failure.
Hold alt key at bootup, if the Hard Drive doesn't show as a boot choice, then you have big problems.
If you don't know the model, find the Serial# & use it on one of these sites, but don't post the Serial# here...
How to find the serial number of your Apple hardware product...
It's possible it may run 10.6.x & you can get that disc cheap from Apple.
Snow Leopard/10.6.x Requirements...
* Mac computer with an Intel processor
* 1GB of memory (I say 4GB at least, more if you can afford it)
* 5GB of available disk space (I say 30GB at least)
* DVD drive for installation
* Some features require a compatible Internet service provider; fees may apply.
* Some features require Apple’s MobileMe service; fees and terms apply.
Which apps work with Mac OS X 10.6?...
Buy Snow Leopard > http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
Call Apple Sales...in the US: 1-800-MY-APPLE. Or Support... 1-800-275-2273
If it's a core Duo & not a Core2Duo, then it'll only run in 32 bit mode.