Assuming both the key and the message have a reasonable amount of entropy, then as Steffen Ullrich said, it reduces to the same level of security as SHA-256 of the input. (The values will not be the same, of course, but it will be the same level of complexity to invert.)
However, if your string is as simple as "hello", then a dictionary attack is quite reasonable. Any operation that uses only low-entropy inputs is susceptible to such an attack: just insert all possible input values and find the one whose output matches.
Using HMAC SHA-256 in the way you've described does not seem to offer any benefits beyond a plain SHA-256. Also please note than hashes and MACs are not "encryption" -- they are supposed to be one-way operations whether or not you have the HMAC key.