Please provide your code...but the most likely is that you do not use a SetDelayed (:=) for the definition of u[n] or that you don't Clear u before redefining it.
In[1]:= Clear[u]
u[0] = 1;
u[n_] := (2^u[n - 1])/n
In[4]:= u /@ Range[0, 4] // N
Out[4]= {1., 2., 2., 1.33333, 0.629961}
In[5]:= Clear[u]
u[0] = 1;
u[n_] := (3^u[n - 1])/n
In[8]:= u /@ Range[0, 4]
Out[8]= {1, 3, 27/2, 531441 Sqrt[3], 3^(531441 Sqrt[3])/4}
Please note that the recurrence with a 3 is quickly divergent (overfow at n=5).
In[9]:= u[4] // N
Out[9]= 2.054631331186877*10^439181
In[10]:= u[5] // N
During evaluation of In[10]:= General::ovfl: Overflow occurred in computation. >>
Out[10]= Overflow[]
