get some iodine, take some of the grains out of the mashtun, put the iodine on them and see if youve fully converted your starch into sugar.
I guarantee, you did not get all the sugars out of those grains. Also, I made this mistake on my first all grain as well, do not fully open the valve on your mashtun. Let the wort slowly pour out. It should take you like 40 minutes to drain the mashtun for a 6-7 gal batch. If you drained it in less than 15 minutes, you had the valve all the way open and were unable to efficiently capture all the sugars. Im sure the beer will still tast delicious, just lighter in flavor and ABV. Remember, less sugar doesnt just mean lower ABV, your losing your gravity too which means your final gravity will be lower as well. You lose your flavor, mouth feel, and ABV.
Id guess it will end up like a light IPA beer. still better than coors light for sure tho. lol. Dont add sugar to bump up the ABV. You wont like it.
For your next all-grain, heat up your mash water to around 168-170*F. Mix it into your grain in the mash tun so that you are at a ratio of 1.5 quarts per Ib of grain. Should be thick like fresh oatmeal, not too thick like oatmeal thats sat out forever, or too thin like oatmeal that has too much water. The grains absorb the heat and lower your mash temp which is what I bet happened to you. 20 Ibs of grain with 168*F water should bring you near 155*F mash temp.
Do these things next time and your next All-grain will go pretty well, at least at the mash phase.