OK, so you are getting a boot chime, fans and monitor all come on and then you get a 'frowny Mac' right?
This suggests either a software problem or possibly a hard drive failure.
To test the software theory, you will need a Mac OS install disc. Anything from 8.6 onward will boot the Blue & White G3 if memory serves. If you value the data on the drive, you will need a disc which is pre-OS X, and you should do a clean install.
Otherwise you can erase the drive and install whatever OS you can find for it. Tiger will most likely require a DVD ROM drive in it though.
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