How to recover stored wifi password?
The hex value you found in the wpa_supplicant.conf is the PMK (not the PSK directly but some intermediate value computing the PSK or something like this). This value is not encrypted, it is hashed as defined by WPA2 using PBKDF2 SHA-1 with 4096 iterations see Wikipedia. 

SHA-1 is a one-way function, which means that it can not be reversed.

Wifi still works, because the PSK is the actual key Wifi requires to work. The passphrase is just for the "dumb humans who can't remember a simple 64 hex chars value" (aka user convenience). 

Hence there is no "decrypt" function. The only way to retrieve the password back is to perform a brute force search on that password (trying all possible passwords). Depending on the length of the password and your hardware (CPU, GPU) this can take some days, weeks, months or years.

There is an issue on Github with indicates that the tool "hashcat" is able to break this value using mode 12000 (-m 12000): 

See also Encryption algorithm used in WPA/WPA2