I had a similar issue making a Rogue Oatmeal Stout clone today. I feel like I either A. Boiled off a TON of the wort, or B. didn't get enough of the water out of the grain bag (held it forever, was barely dripping when I threw it out). I ended up with about 2 gallons of wort, so I added enough water to make it about 4.5 gallons, giving me a 1.052 OG, instead of the 1.060 target.
I'm just going to ferment as usual and hope for the best. I would rather have a better flavored beer than grab an extra 1% of alcohol. Only my 3rd batch, so I'm just aiming for success, not perfection.