my a/c stopped working. what is causing the problem?

Anytime you compromise the AC system including lines or the different components like the compressor or the condenser, drier, etc. you need to basically pull out all remaining air from the system and purge any remaining refrigerant as well. When you replaced that portion of the line a vacuum could of been created and sucked in a bunch of air. Now that you have it all sealed up bring it to a garage that has a AC system Vac or purchase one from Harbor Freight or some place. You might be able to rent one as well at certain parts stores. You may want to replace your Drier (Filter).