If you have had actual damage to your system.
Have it inspected by a professional who will scope (put small camera down" the system to find the internal breakage. Remember that 99% of the time systems are not "broken" and only need to be cleared of compacted sludge and grime that has accumulated over the years.
If your problem is not mechanical, you need bacteria added.
Bacteria acts like acid and will eat through the blockages that have developed. Whatever you do, don't add chemicals!
. Chemicals do nothing more that free up sludge and force it further into your system - making things worse.
Find the strongest most aggressive bacteria dealer you can on yahoo, goggle or msn and order a bit more than the recommended dosage.
There are many companies selling different products for different situations so make sure you find something strong and get it into your system quickly to get the product working.
Allow the bacteria to do its job.
This may take anywhere from a few days to a few weeks depending on how much sludge you have in your tank, lines and field. If you system is very old and has been neglected. The bacteria can take up to 3 weeks but keep in mind, once the bacteria is in the system, it multiplies rapidly and eats everything organic in its path. Bacteria is completely safe and will not harm your system as chemical agents can and often do.
Read about how bacteria works and don't let the repairman tell you that "we" have enough coli-form in our own feces to breakdown our own waste. Not True!
Back in the early days when we didn't use bleach, antibacterial soaps and harsh cleaners, we did actually produce enough bacteria in our own stool to degrade and digest our waste but in modern times, it simply does not hold true. Do your research and find out for yourself.