Powering up.
Answers
No, or at least not at the moment.
I'm guessing you are level 38 or 39 at the moment - you can level a pokemon to 1.5 levels above your trainer level, and a pokemon cannot currently go above level 39. If they ever extend the level caps, the button should re-appear but, for now, there is no further progression possible.
Like the above mentioned, unfortunately, at least this moment, your machamp is maxed at Lv39 and there is no way to further power up. What I heard others do is to power up your mon to the critical point Lv 38.5 first and wait until your trainer level to reach Lv39.
Then make sure you have enough stardust and candy. Prepare 3 cell phones and log in to this power up page. Simultaneously press the power up in the 3 phones. If it succeeds, you will see your mon straightly goes to Lv40. The key behind is that Lv38.5 > 40 requires 3 times level up but the system now looks like only allows one more time to power-up after Lv38.5.