Since priming sugar only creates a small amount of alcohol, if you want to make the beer stronger, you have to do it when you first brew it. Corn sugar can work then, adding alcohol without adding body or flavor, but that won't make for a very good beer in many cases. To be more specific, it's not always bad but it doesn't suit all beers and should be in moderation in any case, only a relatively small percentage of the fermentables. Adding more malt (or malt extract, since I assume that's what you're using) is better, but also makes the beer heavier, requiring more or different hops to balance, and so on.
This all sounds complicated, but don't worry, it isn't really. I've just gone through learning it all quite recently myself. If you're getting ready to bottle I'm afraid it's too late to strengthen your current kit, but for future beers if you want to make something strong just look for a kit or recipe that makes a style of beer you like at the alcohol level you want. The recipe database here is excellent, there are fine books to choose from, and most brewing supply stores will have kits(though some with better quality and variety than others.) Just be warned: stronger beers take longer to ferment, longer to carbonate, and longer to age to perfection than milder ones, so they take more patience. Usually this only becomes really pronounced once you get to really strong beers, but it's a general rule of thumb for most styles.
Reason: Wasn't even drunk, just distracted and missed stuff.