My Macbook Air's Charger will not charge when plugged in, I've already reset the SMC. I also just got it back from Apple...What can I do?
There are several reasons a MagSafe power adapter will intermittently show the Green/Amber LED charge status. If it comes and goes it may indicate a short or break in the thin DC line cable. This usually happens at the power brick end where it runs into the adapter itself. You may not even see a cut in the plastic wire cover but if it looks badly creased it may be a wire break internally that touches enough sometimes to work. The other end (where the MagSafe connector is) can have the same issue. If the MagSafe is the MagSafe 1 type (MagSafe 2 has a thinner connector profile) and it is the right-angled L-shape connector even if the cable looks fine it may be that it has twisted loose inside the metal connector. The cable running into the connector should not twist at all if you hold the metal connector in one hand and gently try to twist the cable running into it with the other hand. If it rotates at all that or a DC line break on the other end near the adapter itself means you need a new adapter. Otherwise try connecting the MagSafe connector the opposite direction you normally connect it and wiggle it when attached so the pins seat correctly. You will know it works if the LED lights.

If after all of this you still have issues and the computer itself in the upper right of the display continues to indicate it's not connected to power it may simply be a bad adapter needing replacement, the connector on the computer needs to be cleaned or replaced (MagSafe DC-in board) or worst case the charging subsystem on the logic board is defective. There are other possibilities but more rare. If a new adapter works consistently then getting a new adapter is your answer. Make sure is is a REAL Apple adapter. There are many many knockoffs that look like the real thing until you look closely. Typically they weigh less and of course cost less but they don't last very long. Hope something here helps.