There are two reasons for the denial. The first is that the SSA indicates that you are not qualified for either the SSDI or the SSI program, or both. This is based on your work history, your household income, and your net worth. Generally, unless you believe they are making an error as to what you paid into the social security system or your income level, this is not something that attorneys generally appeal.
The second is that the SSA determines that you are not medically qualified for benefits–meaning they believe that you are able to perform gainful employment or your medical condition doesn’t qualify. This could be grounds for appeal. There are four levels of appeal at the administrative or agency level. In order of application, you begin by filing an initial claim. You then file for reconsideration, you then go before an administrative law judge for a hearing, and finally, you can appeal the administrative law judge’s decision before the appeals council.
You can be approved or denied at any of those four levels. After you have exhausted all four agency levels, you can either start your application over and remain at the administrative level by filing a new initial claim. Or, you can file an appeal by the appeals counsel leaving the agency level and appealing to the federal district court.