Since brick walls are a constant battle, I want to walk you through a few more scenarios so you will understand how to manage the problem step by step.
Before you can look for missing information, you need to write down what you already know. One way to do this is a timeline.
You can do a very simple timeline like this:
Timeline of Silas Simmons
1794 -- Born in S.C.
1803 -- Approx. year Silas migrated to Mississippi with his family
1814 -- Discharged from the War of 1812 as a private
1818 -- Married Janet Freeman
1820 -- Living in Perry Co, Miss. per census
1830 -- Living in Perry Co, Miss. per census
1840 -- Living in Perry Co, Miss. per census
1841 -- Living in Perry Co, Miss. per state census
1845 -- Living in Perry Co, Miss. per state census
1850 -- Living in Perry Co, Miss. per census
1853 -- Living in Perry Co, Miss. per state census
1855 -- Received a 40 acre land grant in Perry Co, Miss.
There is no record of Silas after 1855.
Looking at this timeline we can see that Silas was 24 years old when he married Janet. It is possible he was married previously and that is something we would need to look at.
On Silas's real timeline I would have added all of his known children. Before birth control, children were spaced approximately two years apart, the first child coming approximately one year from the date of the marriage. If you have gaps in the children's ages, then you have to think about why. There could have been a miscarriage, a child's death or perhaps the father was off working somewhere for an extended period of time. Some of these questions can be answered by following the census records from year to year.
Another way to find the gaps in your information is to write it out longhand.
Silas Simmons (unknown middle name)
Born 1794 in S.C. (unknown exact date and unknown exact location)
Married Janet Freeman in 1816 (unknown exact date and unknown exact location)
Died between 1855-1860 (unknown exact date) in Perry Co, Miss.
I am missing quite a bit of information. Lucky for me Silas didn't move around much, so I will concentrate my efforts in Perry County, Miss. I would need to see how far back Perry County marriages records go. We know that Silas was a land owner by at least 1855. He could have owned other land earlier than that, so I would want to check land and tax records. Perry County might have done their own local censuses which would be helpful. Silas doesn't appear on the 1860 census, so I would need to check the 1860 mortality census in case he died in 1860 exactly.
Take the time to write out what you already know so that you can have a better idea of what information you still need to uncover. Genealogy is like putting together a complex jigsaw puzzle with several pieces missing. It takes time and patience to put things together so that everything finally makes sense.