I wasted a couple of hours on this yesterday. Go to your droplet settings, select the Kernel tab. DO will claim that you have some kind of Grub Loader kernel active — select another kernel from the list. I think any ubuntu will work, but if you select a different one than you had, some of the software might need an upgrade. Mine was
Ubuntu 12.04 x64 3.13.0-32-generic for example.
If you have enough patience to scroll through all that list one of them should in fact be greyed out and selected. You cant choose that one, but select one next to it, hit „Change”, then select back the one you should have and reboot. That fixed it for me.
The other way to do it would be to select any kernel with matching architecture (32 vs x64), reboot, log in and determine what kind of kernel headers were installed on the system, so you can know for sure what kernel to select. YMMV.
Hope this helps.