What do you do if radio communications were lost in the practice area

I'm pretty it has something to do with 7400 but i really don't know>

You would squawk 7600 on your transponder to indicate that you have lost radio communication. It is also important when changing squawk codes that you're careful not to dial through either 7500, the I've-been-hijacked squawk code. You should also avoid dialing through 7700, the emergency squawk code unless you really need to get the controllers attention quickly. Squawking 7700 does set off an audible alarm in any controllers work space whether it be the control tower, approach control or center. Consequently, if you are in a high traffic area in controlled airspace, it might be prudent to set your transponder momentarily to 7700 and then switch to 7600. If you're not in a high traffic area and some distance from the airport, 7600 would suffice. So you would want to reset your transponder squawk code by resetting from right to left. For instance, if you were in your practice area with the VFR squawk of 1200 set on your transponder, you would want to first change the 2 to a 6 and then the 1 to a 7.

If you're flying from a controlled airport (control tower), you would need to know what the tower light signals meant when you returned to land. Take a look at: http://pad39a.com/gene/lg.html for a nicely laid out table of the control tower light signals used by air controllers to safely guide and authorize various clearances for aircraft with radios out of commission.

What is 7400...? I do not know "that one" -

Loss of communication code is 7600 -

If you lost communication, maybe it is the ground station, not you...
Or you cannot transmit but can receive -
So, remain a while on the VHF frequency assigned to you -
If after a while, still no contact, select and try 121.5 and monitor -
If you have 2 COM VHF radios, one should be on 121.5 when flying -