I un plugged my I Mac when i turn on again the fan is now blowing air without stop

this is a signal, that there is a problem on the boot volume. The system make a check routine and, as you boot in single user mode (SUM), the fan starts after a time of (?) 3 minutes in SUM to be sure that the Hardware will not become to be hot. This happens because in SUM the System have not information about the internal temperature of the system; this is given by working system in normal boot mod.