You don't need a loop:
a = rand(3,4)
sum(a) % sum of each column, also sum(a,1)
sum(a,2) % sum of each row
sum(a,'all') % sum of all values, or sum(a(:)) on old releases
If you really want to do it in a loop:
s=0;
for i = 1:numel(a)
s=s+a(i);
end
s
