dear DDHarLiN, I imagine that you're under Windows. Which compiler uses Qt (MinGW 4.4 or MSVC++) ?
If like me you use MinGW 4.4, runs the shell command:
START | Programs | Qt by Nokia v4.8.1 (MinGW OpenSource) | Qt 4.8.1 Command Prompt

then go to the directory of your project to be compiled (where the file is present .pro), then type:

qmake && make