Um.
That string
string string1 = "7210110810811132119111114108100";
Isn't ASCII.
ASCII is a character set, in much the same way as UNICODe is, but a lot smaller. Each character has a value: H would be 72 decimal, or 48 Hex, or (unusually these days) 110 in Octal. Your string may contain some numbers which translate to ASCII values, but it can;t be read as a string of ascii characters because the numbers aren't delimited in any way. OK we can assume its Decimal, becuase we "know" that teh first character is 'H' and it starts with "72", but all of "1", "10", and "101" are value ASCII characters. SOH, LF, and 'e' respectively.
It's possible to convert this string, but really you need to go back to the source and either get the values as 8 bit bytes, or as two hex digit values before you even try.
If you have either, you can read them as an array of bytes, and then its trivial to convert that to a printable Unicode string:
byte[] bytes = ...
string s = System.Text.Encoding.ASCII.GetString(bytes);