File system has become read-only after updating to 15.04?

The solution was posted in the comments by @Lewis Lebentz Jul 26 at 15:00.

I'll paraphrase so anyone looking for the answer can find it here easily. But @Lewis should post the answer himself mark it as answered and you get due credit.

The solution: Open a support ticket, ask Digital Ocean to mount the recovery ISO (It's a special ISO that only they can mount).

1. Choose 1 to mount the filesystem and the edit /etc/fstab. Note: Use the console and run nano or vi /mnt/etc/fstab. Alternatively you can enable SSH and networking (in the recovery options) to login with your terminal (see instruction) though I havn't tried that myself.
2. Changed the UUID in there to the output of blkid, save.
3. Ask DO to remove the recovery disk. Restart and you should have access again!

You can do as ændrük posted in comments:

$ mount -rw -o remount /dev/vda1 /
$ sed s/wrong_uuid/correct_uuid/ -i /etc/fstab

..and then boot your linux again! Make sure you change vda1 with your device name. And in the sed command, the right uuids, of course!

I found this happening to me aswell. The disk UUID in /etc/fstab couldn't be resolved. I fixed this by first finding the UUID of the disk by running

sudo blkid -c /dev/null -o list

And copying the disk UUID for mount point /

I then followed @ændrük comment and remounted the disk with

mount -rw -o remount UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxx

I then edited /etc/fstab to change the disk UUID for the root disk.