The bad news is that it probably isn't worth drinking. It is going to taste like very watered down beer beyond session beer ABV levels.
The good news is that all is not lost! To fix this batch, what you do is brew the same beer again with higher ABV, ferment it out, then BLEND the two. Simple math will tell you what OG you have to hit to get a good blend.
If you have 5.0 gallons at 1.025, and want to up it to, say a blended 10.0 gallons at 1.052, you need to make a second 5 gallon batch at 1.079 to average everything out. Just pump up the grain bill in proportion in BeerSmith, keeping the hops and yeast the same.
Lots of extra work, but it'll solve your problem.