The first step would be to power cycle your router by unplugging it from the power for 30 seconds then plugging it back in.
The second step would be to adjust your antennas. Do this by moving one of the antennas in a 90 degree angle.  By ensuring the antennas are not parallel to each other may be the solution to reaching the problem area. This step however can be skipped if you have a router with built in antennas.
Learn more about how you can boost your wifi signal here.