When you ask for help you'll get it quicker by providing some details beyond "It's Broke!".
Here are some of the most common problems to look at:
1). Are you running a 64-bit machine?
2). Are you using a 64-bit OS?
3). Are you using 64-bit Java? 32-bit Java will run on 64-bit machines, and for some stupid reason still gets installed if you just click the "Download Now" button at Oracle.
All packs older than those based on Minecraft v1.6 run on 32 bit. Some larger 1.6 packs run slower. Not a lot of the 1.7 packs do, and none of the larger ones do, since 32-bit systems have a hardware dictated 3GB ram limit, smaller that the recommended ram allocation, which is 2GB minimum, 4GB maximum. These numbers represent the amount of Ram left after subtracting what the OS needs to run smoothly.
4). Do you have at least 4GB Ram? Your OS will need 1.5-2GB or more, if you have integrated video. GPU cards usually have their own dedicated ram. Java+MC+Mods needs 2.5GB for smaller packs, more for larger ones up to a limit of around 4GB. More ram allocated to the client just causes a variety of lag issues. Java 1.8 (current) automatically allocates 4GB ram maximum by default.
Check these possibilities on your system. If everything looks ok then people will need installation/crashlogs to debug further.