The keg is ruined. Allowing it to freeze is what did it in. All the gas is forced from the liquid, and it does not re-enter it when it thaws. Simply re-applying the pressure will not help anything to do with the lost internal CO2 of the beer.
It still cascades because of the separator at the end of the stout faucet, there is no head and a flat taste because the CO2 has left the beer.
Just a couple things-
If you have already had that keg for a month and your only 25% through it, you really are not drinking it fast enough to keep it fresh and carbonated all the way to the end,- 4 months later.
I may have an August date on it, but that is a freshness date for reference before you tap it. Once you tap it, you really should finish it within 6 weeks, 8 weeks at the most, for top freshness.
Eventhough Guinness may recommend 38 psi, I've always had good luck with it around 30 psi personally.
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