Dendrobium: black leaf tips, then spots and yellowing

[justheretoask]

Hi,

I'm a total beginner in plant care. I've got this orchid, which I assume is a Dendrobium phalaenopsis. The tips of the leaves on one of the stems have started going black. The colour spreads into the leaf gradually, and small black spots appear. Then the tip starts yellowing. What could be causing it?

Could it be temperature-related? Before I spotted this problem, I'd left it next to an open window a couple of times, and it'd been quite cold out.

Thank you.