If it's flat after 7 days at 20 degrees it's almost certain that you have a leak! Sorry.
The usual culprit is the keg cap seal.
There is still a chance to nip it up (I use a wood-clamp) and it may still carbonate sufficiently to serve a few pints before needing to use CO2 capsules.
In future, I recommend that you check that the beer is carbonating after a few days by just running 100ml or so into a glass. If it's not foamy then it ain't carbing up.
If it doesn't carb up this time then you will need to remove the cap, smear Vaseline around the "O" ring and the cap threads, re-prime the keg with (I suggest) no more than 50g of sugar (for a 23 litre brew), re-seal the cap, tighten it up, put the keg back in a nice warm place at 20 degrees and check in a couple of days as per above.
Personally, I always dissolve the sugar in water, bring it to the boil, cover it with tinfoil, let it cool and then add it to the keg and very gently stir it in before closing up the keg. (Just a few grains of sugar around the cap will screw-up the seal and this method avoids that situation.)