How do i lock a cell so that its result doesnt change?

Basically in cell A1 a value of 1-5 will be entered, in cell B1 will be a
value. Cell C1:C5 will list numbers 1-5 the problem is the formula results in
D1:D5 ie D1 will contain the value B1 if A1 = 1 or D2 will contain the value
B1 if A1 = 2 and so on. so if i enter value 1 in A1 and value 20 in B1 (which
then should put the value of 20 in D1 how do i stop the value from D1
disappearing if i change A1 to 2? i have used the copy paste value but would
rather automate it then having to repeat it every time