Each time you start up your Mac after a complete shutdown, the system performs a self-test before booting the operating system. If everything goes fine you'll boot into Mac OS, but if not, the system will emit a set of beeps. After the beeps, the screen will stay black, the system's LED light will flash and you'll be unable to start the operating system. The flashing LED is directly connected to the audible beeps; if you heard 2 beeps you'll see 3 flashes, if you heard 3 beeps you'll see 4 flashes, and so on.
The reason your Mac is beeping at you is because something is wrong with its memory. One beep means no RAM was found at all, two beeps mean the type of RAM installed is incompatible with your system, and three or more beeps could mean serious problems. If you recently installed or upgraded your RAM chips, the probablem could likely be that you installed the wrong chips, inserted them incorrectly or have bad RAM altogether. Consult your system manual or the Apple website for the specific type of memory required by your Mac, and make sure the chip(s) are firmly inserted in the slot(s) and held down by the clips on either side of the slots.
If you have not installed or upgraded your RAM recently, the chips appear to be installed correctly and/or you're just not sure what's wrong, then you should go straight to Apple for help. Either call their tech support number or go to the Apple store and ask the guys at the Genius Bar for help. They'll either fix your problem in the store or, if you have more serious problems, they'll ship it to Apple for repairs. If your problem is simply incorrect or inoperable RAM chips, a new 512 MB chip will cost you from $100 (used) to $150 (new) and the Genius will install it for you on the spot. If your logic board needs to be repaired then this situation may end up costing you upwards of $300, but without the ability to boot up your computer there's really not much you can do aside from repairing it or buying a new one. The Geniuses in the Apple store will let you know exactly what's going on.
Caution! You should never open up your computer unless you're confident that you know what you're doing, especially when dealing with laptops. If you are comfortable with the internal hardware of your system, be sure to follow the instructions that you can find by searching the Apple support website. If you are not comfortable with any of this or have never done it before, leave the repairs to the professionals and take your computer to the Apple Store.