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I just bought a new bike (my first brand new one ever) a few months ago and a while ago I decided to give it a wash and after that my front hydraulic brake hasn't been working as strong as it used too. I used to be able to lock it up on gravel with two fingers, now I can't get them to lock up at all even with all my strength.
(This is my bike) http://www.trekbikes.com/au/en/bikes/mountain/sport/3_series/3900_disc/
Most likely cause if the disc and pads are contaminated. What did you wash it with? Many cleaners leave a residue.
Worst case the pads need replacing. The discs should be well cleaned before installing new pads - Use a solvent such as brake cleaner, methylated spirits or Isoprop alcohol, and rub the discs to be certain no contamination is left on them.
Best case is a good clean of the discs solves the problem. If this does not work, remove the pads and clean with cleaner and re install and clean disk again. If this does not work there are tricks with pads such as burning, sanding etc that might help - I won't cover how to do it here as its well covered elsewhere.
How far you go trying to restore contaminated pads is more a matter of how much time and effort is needed vs cost. Eventually (and quite early for me) patience runs out and as new pads are the guaranteed fix, its down to the LBS I go.
From your link, your bike has Tekro HDC Hydraulic brakes.
If you do a google search, there is a video (made by Tekro by the looks of things) which talks about how to replace pads on these brakes.
I suggest you watch this video. If you understand it, and feel comfortable doing what it suggests, take the pads out and examine them. Don't buy any replacements just yet - fingers crossed, simply taking the pads out and putting them back in might solve your problem. Also read up on the pistons, try to get an idea how they actually cause the brake to work, and satisfy yourself that all is well.
If you're not comfortable with any or all of this, take the bike to your local shop.
My first encounter with hydraulic brakes, I ummed and erred for ages because it was something new to me, when I finally found the courage it was quite a straightforward job - I did it in front of the laptop watching one of these vids, pausing it frequently so as lot to let them get too far ahead.
I'd suggest cleaning the rotors with acetone (nail polish remover). Not the colored kind with fragrance and other things in it but pure, clear acetone. It's a bit more aggressive than isopropyl alcohol or ethanol and has worked better and faster in my own cleaning efforts. It's also cheap.
While you're at it, I'd remove the brake pads, clean them as well, and then use a fine sandpaper to rough up the surface of the pads (scrape perpendicular to the direction of the rotor movement) and expose a new layer that is (hopefully) free of contamination.
If that doesn't do anything, I'd bleed any air that is in the hydraulic system. Air can compress and thus weaken the hydraulic force transferred to the calipers, whereas the mineral oil or other hydraulic fluid you're using is not compressible.
