They may be incompatible. I would think that they'd work. Let's clarify here, what RAM are you using?
2x 2GB 667MHz modules of the same brand/model could work in a 533 RAM slot.
2x modules at the same MHz from different brands may work together BUT mixing RAM is not great idea, it could possibly cause damage to your system it much less have trouble booting/stability issues. You could get away with it though, sometimes it has just worked for me.
2x modules from different brands at different MHz in the same board that doesn't technically take that MHz RAM is nearly guaranteed to cause issues somewhere.
Try each module individually then, if you dare, try them in different combinations. If it doesn't boot (at least make it to BIOS) with the original module only than there is now a problem in your hardware. It may be damaged RAM or slots but hope that it's something benign. Check the cords/connections around
your board in case you knocked something loose.