Why does my while loop only run once?

Question

I want to make this loop run more than once but it will only run once and then stop. What am I doing wrong?

a = 0
Let = input("Enter a sentence to decrypt : ")
Num = 0
b = 0
while b < 26 :
    Num = Num + 1
    num = int(Num)
    while a < len(Let) :
        if  ord(Let[a]) > 96 and ord(Let[a]) < 123 and ord(Let[a]) + Num > 96 and ord(Let[a]) + Num < 123 : 
            print(chr(ord(Let[a]) + Num))
        elif ord(Let[a]) > 64 and ord(Let[a]) < 91 and ord(Let[a]) + Num > 64 and ord(Let[a]) + Num < 91 : 
            print(chr(ord(Let[a]) + Num))
        elif ord(Let[a]) > 96 and ord(Let[a]) < 123 and ord(Let[a]) + Num >= 123 :
            while ord(Let[a]) + num >= 123 :
                    num = Num - 26
            print(chr(ord(Let[a]) + num))
        elif ord(Let[a]) > 64 and ord(Let[a]) < 91 and ord(Let[a]) + Num >= 91 :
            while ord(Let[a]) + num >= 91 :
                    num = Num - 26
            print(chr(ord(Let[a]) + num))
        a = a + 1
    b = b + 1

Answer 1

"a" is not being reset to 0 in the "b" loop:

Let = input("Enter a sentence to decrypt : ")
Num = 0
b = 0
while b < 26 :
    Num = Num + 1
    num = int(Num)
    a = 0
    out = ""
    while a < len(Let) :
        if  ord(Let[a]) > 96 and ord(Let[a]) < 123 and ord(Let[a]) + Num > 96 and ord(Let[a]) + Num < 123 : 
            out += (chr(ord(Let[a]) + Num))
        elif ord(Let[a]) > 64 and ord(Let[a]) < 91 and ord(Let[a]) + Num > 64 and ord(Let[a]) + Num < 91 : 
            out += (chr(ord(Let[a]) + Num))
        elif ord(Let[a]) > 96 and ord(Let[a]) < 123 and ord(Let[a]) + Num >= 123 :
            while ord(Let[a]) + num >= 123 :
                    num = Num - 26
            out += (chr(ord(Let[a]) + num))
        elif ord(Let[a]) > 64 and ord(Let[a]) < 91 and ord(Let[a]) + Num >= 91 :
            while ord(Let[a]) + num >= 91 :
                    num = Num - 26
            out += (chr(ord(Let[a]) + num))
        a = a + 1
    print(out)
    b = b + 1

Changed to a "for" loop:

Let = input("Enter a sentence to decrypt : ")
Num = 0
b = 0
while b < 26 :
    Num = Num + 1
    num = int(Num)
    out = ""
    for a in Let:
        if  ord(a) > 96 and ord(a) < 123 and ord(a) + Num > 96 and ord(a) + Num < 123 : 
            out += (chr(ord(a) + Num))
        elif ord(a) > 64 and ord(a) < 91 and ord(a) + Num > 64 and ord(a) + Num < 91 : 
            out += (chr(ord(a) + Num))
        elif ord(a) > 96 and ord(a) < 123 and ord(a) + Num >= 123 :
            while ord(a) + num >= 123 :
                    num = Num - 26
            out += (chr(ord(a) + num))
        elif ord(a) > 64 and ord(a) < 91 and ord(a) + Num >= 91 :
            while ord(a) + num >= 91 :
                    num = Num - 26
            out += (chr(ord(a) + num))
    print(out)
    b += 1